Rikki

Thanks, I have moved this to a bug report - we'll get it fixed.

Thanks, I have moved this to a bug report - we'll get it fixed.

4.5 introduces some changes to CSS, so I wanted to provide an overview of those and how they might affect you.
 
IE11
Firstly, we've completely dropped support for IE11. This means you'll start to see newer CSS methods being used that IE11 does not support. 
 
Atomic classnames
One thing you'll notice as you read on is we're moving towards using atomic classnames for utility styles. What does that mean? Basically, each classname is just a convenience helper for applying one particular style. You build how an element looks and behaves by applying multiple classnames. 
While this is slightly more verbose in the HTML, it's also much clearer and avoids having to create mutant selectors that exclude certain elements. If you see an element with ipsBorder_top, it's obvious what it does. Don't want a top border? Just remove that class.
Another benefit is that it allows more precise control over how styles are applied at different device sizes, because individual styles can be modified. Want a top border on desktop, but not phones? ipsBorder_top sm:ipsBorder:none will do the job - no crazy selectors or additional media queries necessary.
 
Class prefixes/modifiers
Wait, what's that sm: bit in the atomic classname example above? That's a new naming convention you'll see which controls at which breakpoints the style is applied.
Unprefixed (e.g. ipsBorder_top). Applies to all devices, desktop and smaller. md: (e.g. md:ipsBorder_top). Applies to tablets and smaller. sm: (e.g. sm:ipsBorder_top). Applies to phones only. sm and md prefixes take priority over the desktop classname. This means you provide the desktop classname but can override it for tablets and/or phones by also adding an md: or sm: classname. For example, ipsPadding_top:double sm:ipsPadding_top:half applies double top padding on desktop, but only half top padding on phones.
You'll also see modifiers on some classes (as in the example above), for example ipsPadding:half. ipsPadding is saying 'apply padding to all sides', and the ':half' modifier makes it a smaller amount.
 
BEM classnames for modules
While we're using atomic classes for global utility styles, we're moving towards BEM for module-specific classes. BEM is simply a naming convention, so it doesn't have too much impact on you - you'll just see a new structure for new classnames that should be easier to understand. 
 
Note: We haven't rewritten our CSS framework with the atomic classname, class prefix/modifier or BEM approaches. Don't worry - 90% of the classes you're used to will be the same. I just wanted to point out that going forward, new classes will follow these paradigms.
 
CSS Variables & calc
We're now making use of CSS variables (also known as CSS custom properties). Check out this MDN article if you aren't familiar with them. This enables us to define some values in one place but use them throughout the product - and it allows you to change the value in one place if you wish to do so for your theme. Most of our CSS variables are defined at the top of global.css in the CSS framework, but you'll also see some other local variables defined in other places. Variables are simply used by wrapping the name in the var() function, e.g. var(--positive-light). 
You'll see some variables named --sp-1 and so on. This is our new 4px spacing scale. In 4.5 and going forward, when we style elements we'll generally be using one of these values for widths, heights, borders, spacing etc. to keep everything consistent. You should do the same for elements you create.
We're also making use of calc(). This is another CSS function that allows math operations to be done. Instead of having to hardcode numbers for positions, sizes etc., we can now use calc() to do it for us based on some other values (often CSS variables).
CSS Variables for theme settings
We are deprecating the usage of the {theme} and {hextorgb} tags for color-type theme setting keys (but not for non-color settings or when you need to pass a specific hex code in).
Instead, color-type theme settings will automatically have a CSS variable created for them, named --theme-setting_key, where setting_key is the key of the setting as defined in the AdminCP. The variable will be a triplet representing the color, for example 255, 255, 255. Therefore, this value can be used with both the rgb and rgba CSS color functions.
Here's an example. If in the past you'd wanted to use the area_background theme setting in your CSS, you'd do background: {theme="area_background"}. Or if you want some opacity, you'd do background: {hextorgb="area_background" opacity="0.2"}.
In 4.5, the correct way of using these will be: background: rgb( var(--theme-area_background) ) and background: rgba( var(--theme-area_background), 0.2 ) respectively.
We're doing this now because it will open up some exciting functionality in future. To be clear, any existing usage of {theme} and {hextorgb} will continue to work fine in 4.5, but we encourage you to move over to the CSS variable approach.
 
Hardcoded hex values
In 4.5 we have largely removed all hardcoded hex colors from our CSS files, and adjusted styles to use theme setting values instead. This will make it much easier for admins to fully colorize their theme without hardcoded colors messing things up. I encourage you to update your app's CSS to follow this approach.
 
Font sizes
We've moved font sizes to a fixed scale. These have been implemented as theme settings so they are customizable. However, rather than use the theme setting value directly, you should make use of the new {fontsize} plugin. This plugin ensures the global scale is applied to any values passed in, allowing 'large print' versions of themes to be easily created. You should use the {fontsize} plugin for font sizes both when you use one of the theme settings and when you use specific pixel values (e.g. {fontsize="72"} - for 72px text)
When used with the {fontsize} plugin, the type scale keys are:
x_small (12px) small (13px) medium (14px) base (16px) large (18px) x_large (20px) 2x_large (24px) 3x_large (30px) 4x_large (36px)  
Flexbox
While we've used flexbox in some places in previous versions, 4.5 makes much wider use of it and also introduces a new family of classes. If you aren't familiar with flexbox, I highly recommend this CSSTricks article for a primer on it. Essentially, instead of positioning elements using floats/clears/etc., flexbox treats the container as a flexible box with properties for controlling how elements inside of it as laid out.
4.5 has a number of new classes that are essentially just convenience for the usual CSS rules.
ipsFlex (sets element to display: flex) ipsFlex-ai:start, ipsFlex-ai:center, ipsFlex-ai:end, ipsFlex-ai:stretch (ai - values for align-items property) ipsFlex-as:start, ipsFlex-as:center, ipsFlex-as:end, ipsFlex-as:stretch (as - values for align-self property) ipsFlex-jc:start, ipsFlex-jc:center, ipsFlex-jc:end, ipsFlex-jc:around, ipsFlex-jc:between (jc - values for justify-content property) ipsFlex-fd:column, ipsFlex-fd:row, ipsFlex-fd:column-reverse, ipsFlex-fd:row-reverse (fd - values for flex-direction property) ipsFlex-fw:wrap, ipsFlex-fw:nowrap, ipsFlex-fw:wrap-reverse (fw - values for flex-wrap property) ipsFlex-flex:10 - sets flex-grow: 1 and flex-shrink: 0 ipsFlex-flex:11 - sets flex-grow: 1 and flex-shrink: 1 ipsFlex-flex:01 - sets flex-grow: 0 and flex-shrink: 1 ipsFlex-flex:00 - sets flex-grow: 0 and flex-shrink: 0  
All of these classes have md and sm prefixed versions too, and this opens up the possibility of having different layouts on different device sizes in a way that's much easier than the hoops you'd have to jump through before. For example, to create some elements that show as a row on desktop but collapse to a column on mobile, you'd just apply ipsFlex ipsFlex-fd:row sm:ipsFlex-fd:column. The sm:ipsFlex-fd:column class overrules the ipsFlex-fd:row class on mobile, adjusting the layout. (Note: flex-direction: row is the CSS default direction anyway, so you can actually leave out ipsFlex-fd:row - it's implicit. I included it in the example for clarity.)
 
Padding/margin
We've added new spacing classes for padding and margin, to allow for atomic classnames, device prefixes and modifiers.
ipsPad, ipsPad_double, ipsPad_half, and all of the ipsSpacer_* classes are now deprecated. You'll still see them in our templates and they'll still work in yours, but don't use them in any new work - you should use the updated classes below.
The padding classes are now named ipsPadding:
ipsPadding, ipsPadding:none, ipsPadding:half, ipsPadding:double - apply padding to all four sides ipsPadding_vertical, ipsPadding_vertical:none, ipsPadding_vertical:half, ipsPadding_vertical:double - apply padding to top and bottom ipsPadding_horizontal, ipsPadding_horizontal:none, ipsPadding_horizontal:half, ipsPadding_horizontal:double - apply padding to left and right ipsPadding_left, ipsPadding_left:none, ipsPadding_left:half, ipsPadding_left:double - apply padding to the left side (RTL aware) ipsPadding_right, ipsPadding_right:none, ipsPadding_right:half, ipsPadding_right:double - apply padding to the right side (RTL aware) ipsPadding_top, ipsPadding_top:none, ipsPadding_top:half, ipsPadding_top:double - apply padding to the top side ipsPadding_bottom, ipsPadding_bottom:none, ipsPadding_bottom:half, ipsPadding_bottom:double - apply padding to the bottom side These classes have md and sm prefixed versions too, allowing you to apply different padding depending on the device size.
One side note here: with the old padding classes, padding was simply halved on phones with no opt-out. That's not the case with the new family - if you want half-padding on mobile on an element, you should apply sm:ipsPadding:half in addition to the normal ipsPadding class, for example. This gives you much more control than you previously had.
Margins follow basically an identical pattern to padding, with the same variation of classes, except the name is ipsMargin.
 
Gaps
Another new family of classes is ipsGap. These classes are used when you want spacing between elements. In the past, you'd have to use :last-child or :first-child to exclude an element, or loop over the elements in the template to leave off a class. If elements wrapped to a new line, putting spacing between the lines was tricky too.
ipsGap solves this by applying even spacing between elements, then applying a negative margin on the whole container to bring it back to the starting position.
The classname is followed by a modifier, which is a number from our spacing scale, e.g. 1 is 4px spacing, 2 is 8px spacing and so on.
ipsGap:1 (1-5 available) - applies both horizontal and vertical spacing around each element in the container ipsGap_row:1 (1-5 available, as well as 0 to remove) - applies vertical spacing on each element in the container Notice ipsGap_row also supports the :0 modifier. This allows you to have horizontal-only spacing - simply apply ipsGap:1 ipsGap_row:0.
Be aware that using both ipsMargin (or custom styles that apply a margin) and ipsGap on the same element can cause issues. You may want add a wrapper element to handle your margin in this situation.
 
Borders
We've also added a class family to add light grey 1px borders to elements - used commonly as dividers between some parts of the page.
ipsBorder - apply border to all sides ipsBorder:none - remove border from all sides ipsBorder_vertical - apply border to top and bottom ipsBorder_horizontal - apply border to left and right ipsBorder_top, ipsBorder_bottom, ipsBorder_left, ipsBorder_right - apply border to a particular side These classes have md and sm prefixed versions too, to control borders shown on each device size. This is particularly useful if you apply a border to a flex child which is in a row on desktop but a column on desktop, for example - you will be able to easily control which side the border appears on once collapsed.
 
"Pull" class
To better display content areas on mobile, a class named ipsResponsive_pull has been added which 'pulls' a box on the left and right sides on small devices. It's intended to be used on boxes (normally with the ipsBox class) that you want to take up the whole screen width on mobile, allowing better usage of the available screen space.
 
Template changes
We've worked to keep template changes as minimal as possible, but in an update the size of 4.5 there are still quite a number of changes. Whether these impact you will depend on if you've modified the template (for theme designers) or rely on a particular selector for theme hooks (for developers).
One area that has received fairly big changes is the post/comment templates. We have redesigned the headers and footers of these templates and moved some elements into a separate parent element on mobile devices.
As usual, full template changes will be available once we've released betas.

Bear in mind, you don't have to update styles in custom.css. But if you do want to use the new approach:
All theme settings that are color fields will automatically be set up as CSS variables, so you don't need to do that yourself (the hex will automatically be converted to an rgb triplet too). So if your "mass_color_main" theme setting is a color field, the --theme-mass_color_main CSS variable will automatically exist. So in your first example, your updated code would be correct and you wouldn't need to do anything extra for that to work.
For your second example, are you saying you'd have a theme setting that's just a plain text field? If so, that won't automatically be created as a CSS variable. You'd need to do that yourself, like so:
:root {
    --your_custom_setting: {theme="your_custom_setting"};
}
You can then use that variable however you wish. In this second example, nothing is automatic, so you can name your CSS variable whatever you want.

It won't be any time soon, if ever - the {theme} tag is still used for non-color settings for example. It'd be preferred to use the CSS variable approach for consistency and potentially future compatibility with new features, but it isn't a huge deal right now.
That said, converting them should be easy in most cases. We did our entire framework in a few steps using find/replace in a text editor.

We only automatically convert colors primarily because they make the most sense as CSS variables. CSS variables also have to be valid CSS values, so we can't easily convert all theme settings into CSS variables. If you only use a theme setting in a couple of places, it probably won't be worth making it a CSS variable - just use the theme setting as before.
You are correct that the font size plugin converts to px. If you need other units, simply use them directly, or make them a CSS variable - but you can't pass the CSS variable into the font size plugin like your example. Instead just use them directly; assuming the value of your theme setting was "3rem", you could do font-size: var(--theme-your_setting);.
For your last example, remember the variable value has to be a valid CSS value. So the --theme-my_line_margin_top variable should include the % symbol - you can't concatenate it like you've done.
Based on the examples you've described, I would just continue using your theme settings as before.
 
 

Typographer’s nightmare. 

Many people’s idea of what form a ‘community’ takes has changed significantly from the 2000s (and even 2010s). Invision Community started out as a humble forum software, but I think a good future for the platform would be to evolve towards something more familiar to today’s audience.
I think some progress has been made for sure, with Activity Feeds, Clubs and Status Updates bolting-on some level of ‘social media’ component to the platform. But it still feels like a Forum software that’s wearing a nice pair of Twitter-like shoes. It doesn’t yet feel like an all-in-one social media-like community platform that I feel like IPS is wanting it to be.
That’s not to say I want IPS to completely turn IC into Facebook or something. But there are learnings from ease-of-use, timeline feeds, profile pages etc that can be found from various social platforms that should be adopted (for example, an Invision Community member’s profile page doesn’t look all that fresh and interesting next to a Twitter user’s profile, especially when viewing on mobile where a user has to scroll through an entire sidebar of meta info before reaching any user content 😴 ).
Forums can - and should - still be a core component of the platform, but I think the time is right now to focus on the other apps and build a more holistic community solution - in particular, ***PAGES*** 😄. Re-conceptualising these apps and how they connect and work together to suit a more cohesive ‘social community’ platform is where I think IPS has its best shot of forging a great path for the future.

4.5 introduces some changes to CSS, so I wanted to provide an overview of those and how they might affect you.
 
IE11
Firstly, we've completely dropped support for IE11. This means you'll start to see newer CSS methods being used that IE11 does not support. 
 
Atomic classnames
One thing you'll notice as you read on is we're moving towards using atomic classnames for utility styles. What does that mean? Basically, each classname is just a convenience helper for applying one particular style. You build how an element looks and behaves by applying multiple classnames. 
While this is slightly more verbose in the HTML, it's also much clearer and avoids having to create mutant selectors that exclude certain elements. If you see an element with ipsBorder_top, it's obvious what it does. Don't want a top border? Just remove that class.
Another benefit is that it allows more precise control over how styles are applied at different device sizes, because individual styles can be modified. Want a top border on desktop, but not phones? ipsBorder_top sm:ipsBorder:none will do the job - no crazy selectors or additional media queries necessary.
 
Class prefixes/modifiers
Wait, what's that sm: bit in the atomic classname example above? That's a new naming convention you'll see which controls at which breakpoints the style is applied.
Unprefixed (e.g. ipsBorder_top). Applies to all devices, desktop and smaller. md: (e.g. md:ipsBorder_top). Applies to tablets and smaller. sm: (e.g. sm:ipsBorder_top). Applies to phones only. sm and md prefixes take priority over the desktop classname. This means you provide the desktop classname but can override it for tablets and/or phones by also adding an md: or sm: classname. For example, ipsPadding_top:double sm:ipsPadding_top:half applies double top padding on desktop, but only half top padding on phones.
You'll also see modifiers on some classes (as in the example above), for example ipsPadding:half. ipsPadding is saying 'apply padding to all sides', and the ':half' modifier makes it a smaller amount.
 
BEM classnames for modules
While we're using atomic classes for global utility styles, we're moving towards BEM for module-specific classes. BEM is simply a naming convention, so it doesn't have too much impact on you - you'll just see a new structure for new classnames that should be easier to understand. 
 
Note: We haven't rewritten our CSS framework with the atomic classname, class prefix/modifier or BEM approaches. Don't worry - 90% of the classes you're used to will be the same. I just wanted to point out that going forward, new classes will follow these paradigms.
 
CSS Variables & calc
We're now making use of CSS variables (also known as CSS custom properties). Check out this MDN article if you aren't familiar with them. This enables us to define some values in one place but use them throughout the product - and it allows you to change the value in one place if you wish to do so for your theme. Most of our CSS variables are defined at the top of global.css in the CSS framework, but you'll also see some other local variables defined in other places. Variables are simply used by wrapping the name in the var() function, e.g. var(--positive-light). 
You'll see some variables named --sp-1 and so on. This is our new 4px spacing scale. In 4.5 and going forward, when we style elements we'll generally be using one of these values for widths, heights, borders, spacing etc. to keep everything consistent. You should do the same for elements you create.
We're also making use of calc(). This is another CSS function that allows math operations to be done. Instead of having to hardcode numbers for positions, sizes etc., we can now use calc() to do it for us based on some other values (often CSS variables).
CSS Variables for theme settings
We are deprecating the usage of the {theme} and {hextorgb} tags for color-type theme setting keys (but not for non-color settings or when you need to pass a specific hex code in).
Instead, color-type theme settings will automatically have a CSS variable created for them, named --theme-setting_key, where setting_key is the key of the setting as defined in the AdminCP. The variable will be a triplet representing the color, for example 255, 255, 255. Therefore, this value can be used with both the rgb and rgba CSS color functions.
Here's an example. If in the past you'd wanted to use the area_background theme setting in your CSS, you'd do background: {theme="area_background"}. Or if you want some opacity, you'd do background: {hextorgb="area_background" opacity="0.2"}.
In 4.5, the correct way of using these will be: background: rgb( var(--theme-area_background) ) and background: rgba( var(--theme-area_background), 0.2 ) respectively.
We're doing this now because it will open up some exciting functionality in future. To be clear, any existing usage of {theme} and {hextorgb} will continue to work fine in 4.5, but we encourage you to move over to the CSS variable approach.
 
Hardcoded hex values
In 4.5 we have largely removed all hardcoded hex colors from our CSS files, and adjusted styles to use theme setting values instead. This will make it much easier for admins to fully colorize their theme without hardcoded colors messing things up. I encourage you to update your app's CSS to follow this approach.
 
Font sizes
We've moved font sizes to a fixed scale. These have been implemented as theme settings so they are customizable. However, rather than use the theme setting value directly, you should make use of the new {fontsize} plugin. This plugin ensures the global scale is applied to any values passed in, allowing 'large print' versions of themes to be easily created. You should use the {fontsize} plugin for font sizes both when you use one of the theme settings and when you use specific pixel values (e.g. {fontsize="72"} - for 72px text)
When used with the {fontsize} plugin, the type scale keys are:
x_small (12px) small (13px) medium (14px) base (16px) large (18px) x_large (20px) 2x_large (24px) 3x_large (30px) 4x_large (36px)  
Flexbox
While we've used flexbox in some places in previous versions, 4.5 makes much wider use of it and also introduces a new family of classes. If you aren't familiar with flexbox, I highly recommend this CSSTricks article for a primer on it. Essentially, instead of positioning elements using floats/clears/etc., flexbox treats the container as a flexible box with properties for controlling how elements inside of it as laid out.
4.5 has a number of new classes that are essentially just convenience for the usual CSS rules.
ipsFlex (sets element to display: flex) ipsFlex-ai:start, ipsFlex-ai:center, ipsFlex-ai:end, ipsFlex-ai:stretch (ai - values for align-items property) ipsFlex-as:start, ipsFlex-as:center, ipsFlex-as:end, ipsFlex-as:stretch (as - values for align-self property) ipsFlex-jc:start, ipsFlex-jc:center, ipsFlex-jc:end, ipsFlex-jc:around, ipsFlex-jc:between (jc - values for justify-content property) ipsFlex-fd:column, ipsFlex-fd:row, ipsFlex-fd:column-reverse, ipsFlex-fd:row-reverse (fd - values for flex-direction property) ipsFlex-fw:wrap, ipsFlex-fw:nowrap, ipsFlex-fw:wrap-reverse (fw - values for flex-wrap property) ipsFlex-flex:10 - sets flex-grow: 1 and flex-shrink: 0 ipsFlex-flex:11 - sets flex-grow: 1 and flex-shrink: 1 ipsFlex-flex:01 - sets flex-grow: 0 and flex-shrink: 1 ipsFlex-flex:00 - sets flex-grow: 0 and flex-shrink: 0  
All of these classes have md and sm prefixed versions too, and this opens up the possibility of having different layouts on different device sizes in a way that's much easier than the hoops you'd have to jump through before. For example, to create some elements that show as a row on desktop but collapse to a column on mobile, you'd just apply ipsFlex ipsFlex-fd:row sm:ipsFlex-fd:column. The sm:ipsFlex-fd:column class overrules the ipsFlex-fd:row class on mobile, adjusting the layout. (Note: flex-direction: row is the CSS default direction anyway, so you can actually leave out ipsFlex-fd:row - it's implicit. I included it in the example for clarity.)
 
Padding/margin
We've added new spacing classes for padding and margin, to allow for atomic classnames, device prefixes and modifiers.
ipsPad, ipsPad_double, ipsPad_half, and all of the ipsSpacer_* classes are now deprecated. You'll still see them in our templates and they'll still work in yours, but don't use them in any new work - you should use the updated classes below.
The padding classes are now named ipsPadding:
ipsPadding, ipsPadding:none, ipsPadding:half, ipsPadding:double - apply padding to all four sides ipsPadding_vertical, ipsPadding_vertical:none, ipsPadding_vertical:half, ipsPadding_vertical:double - apply padding to top and bottom ipsPadding_horizontal, ipsPadding_horizontal:none, ipsPadding_horizontal:half, ipsPadding_horizontal:double - apply padding to left and right ipsPadding_left, ipsPadding_left:none, ipsPadding_left:half, ipsPadding_left:double - apply padding to the left side (RTL aware) ipsPadding_right, ipsPadding_right:none, ipsPadding_right:half, ipsPadding_right:double - apply padding to the right side (RTL aware) ipsPadding_top, ipsPadding_top:none, ipsPadding_top:half, ipsPadding_top:double - apply padding to the top side ipsPadding_bottom, ipsPadding_bottom:none, ipsPadding_bottom:half, ipsPadding_bottom:double - apply padding to the bottom side These classes have md and sm prefixed versions too, allowing you to apply different padding depending on the device size.
One side note here: with the old padding classes, padding was simply halved on phones with no opt-out. That's not the case with the new family - if you want half-padding on mobile on an element, you should apply sm:ipsPadding:half in addition to the normal ipsPadding class, for example. This gives you much more control than you previously had.
Margins follow basically an identical pattern to padding, with the same variation of classes, except the name is ipsMargin.
 
Gaps
Another new family of classes is ipsGap. These classes are used when you want spacing between elements. In the past, you'd have to use :last-child or :first-child to exclude an element, or loop over the elements in the template to leave off a class. If elements wrapped to a new line, putting spacing between the lines was tricky too.
ipsGap solves this by applying even spacing between elements, then applying a negative margin on the whole container to bring it back to the starting position.
The classname is followed by a modifier, which is a number from our spacing scale, e.g. 1 is 4px spacing, 2 is 8px spacing and so on.
ipsGap:1 (1-5 available) - applies both horizontal and vertical spacing around each element in the container ipsGap_row:1 (1-5 available, as well as 0 to remove) - applies vertical spacing on each element in the container Notice ipsGap_row also supports the :0 modifier. This allows you to have horizontal-only spacing - simply apply ipsGap:1 ipsGap_row:0.
Be aware that using both ipsMargin (or custom styles that apply a margin) and ipsGap on the same element can cause issues. You may want add a wrapper element to handle your margin in this situation.
 
Borders
We've also added a class family to add light grey 1px borders to elements - used commonly as dividers between some parts of the page.
ipsBorder - apply border to all sides ipsBorder:none - remove border from all sides ipsBorder_vertical - apply border to top and bottom ipsBorder_horizontal - apply border to left and right ipsBorder_top, ipsBorder_bottom, ipsBorder_left, ipsBorder_right - apply border to a particular side These classes have md and sm prefixed versions too, to control borders shown on each device size. This is particularly useful if you apply a border to a flex child which is in a row on desktop but a column on desktop, for example - you will be able to easily control which side the border appears on once collapsed.
 
"Pull" class
To better display content areas on mobile, a class named ipsResponsive_pull has been added which 'pulls' a box on the left and right sides on small devices. It's intended to be used on boxes (normally with the ipsBox class) that you want to take up the whole screen width on mobile, allowing better usage of the available screen space.
 
Template changes
We've worked to keep template changes as minimal as possible, but in an update the size of 4.5 there are still quite a number of changes. Whether these impact you will depend on if you've modified the template (for theme designers) or rely on a particular selector for theme hooks (for developers).
One area that has received fairly big changes is the post/comment templates. We have redesigned the headers and footers of these templates and moved some elements into a separate parent element on mobile devices.
As usual, full template changes will be available once we've released betas.

4.5 introduces some changes to CSS, so I wanted to provide an overview of those and how they might affect you.
 
IE11
Firstly, we've completely dropped support for IE11. This means you'll start to see newer CSS methods being used that IE11 does not support. 
 
Atomic classnames
One thing you'll notice as you read on is we're moving towards using atomic classnames for utility styles. What does that mean? Basically, each classname is just a convenience helper for applying one particular style. You build how an element looks and behaves by applying multiple classnames. 
While this is slightly more verbose in the HTML, it's also much clearer and avoids having to create mutant selectors that exclude certain elements. If you see an element with ipsBorder_top, it's obvious what it does. Don't want a top border? Just remove that class.
Another benefit is that it allows more precise control over how styles are applied at different device sizes, because individual styles can be modified. Want a top border on desktop, but not phones? ipsBorder_top sm:ipsBorder:none will do the job - no crazy selectors or additional media queries necessary.
 
Class prefixes/modifiers
Wait, what's that sm: bit in the atomic classname example above? That's a new naming convention you'll see which controls at which breakpoints the style is applied.
Unprefixed (e.g. ipsBorder_top). Applies to all devices, desktop and smaller. md: (e.g. md:ipsBorder_top). Applies to tablets and smaller. sm: (e.g. sm:ipsBorder_top). Applies to phones only. sm and md prefixes take priority over the desktop classname. This means you provide the desktop classname but can override it for tablets and/or phones by also adding an md: or sm: classname. For example, ipsPadding_top:double sm:ipsPadding_top:half applies double top padding on desktop, but only half top padding on phones.
You'll also see modifiers on some classes (as in the example above), for example ipsPadding:half. ipsPadding is saying 'apply padding to all sides', and the ':half' modifier makes it a smaller amount.
 
BEM classnames for modules
While we're using atomic classes for global utility styles, we're moving towards BEM for module-specific classes. BEM is simply a naming convention, so it doesn't have too much impact on you - you'll just see a new structure for new classnames that should be easier to understand. 
 
Note: We haven't rewritten our CSS framework with the atomic classname, class prefix/modifier or BEM approaches. Don't worry - 90% of the classes you're used to will be the same. I just wanted to point out that going forward, new classes will follow these paradigms.
 
CSS Variables & calc
We're now making use of CSS variables (also known as CSS custom properties). Check out this MDN article if you aren't familiar with them. This enables us to define some values in one place but use them throughout the product - and it allows you to change the value in one place if you wish to do so for your theme. Most of our CSS variables are defined at the top of global.css in the CSS framework, but you'll also see some other local variables defined in other places. Variables are simply used by wrapping the name in the var() function, e.g. var(--positive-light). 
You'll see some variables named --sp-1 and so on. This is our new 4px spacing scale. In 4.5 and going forward, when we style elements we'll generally be using one of these values for widths, heights, borders, spacing etc. to keep everything consistent. You should do the same for elements you create.
We're also making use of calc(). This is another CSS function that allows math operations to be done. Instead of having to hardcode numbers for positions, sizes etc., we can now use calc() to do it for us based on some other values (often CSS variables).
CSS Variables for theme settings
We are deprecating the usage of the {theme} and {hextorgb} tags for color-type theme setting keys (but not for non-color settings or when you need to pass a specific hex code in).
Instead, color-type theme settings will automatically have a CSS variable created for them, named --theme-setting_key, where setting_key is the key of the setting as defined in the AdminCP. The variable will be a triplet representing the color, for example 255, 255, 255. Therefore, this value can be used with both the rgb and rgba CSS color functions.
Here's an example. If in the past you'd wanted to use the area_background theme setting in your CSS, you'd do background: {theme="area_background"}. Or if you want some opacity, you'd do background: {hextorgb="area_background" opacity="0.2"}.
In 4.5, the correct way of using these will be: background: rgb( var(--theme-area_background) ) and background: rgba( var(--theme-area_background), 0.2 ) respectively.
We're doing this now because it will open up some exciting functionality in future. To be clear, any existing usage of {theme} and {hextorgb} will continue to work fine in 4.5, but we encourage you to move over to the CSS variable approach.
 
Hardcoded hex values
In 4.5 we have largely removed all hardcoded hex colors from our CSS files, and adjusted styles to use theme setting values instead. This will make it much easier for admins to fully colorize their theme without hardcoded colors messing things up. I encourage you to update your app's CSS to follow this approach.
 
Font sizes
We've moved font sizes to a fixed scale. These have been implemented as theme settings so they are customizable. However, rather than use the theme setting value directly, you should make use of the new {fontsize} plugin. This plugin ensures the global scale is applied to any values passed in, allowing 'large print' versions of themes to be easily created. You should use the {fontsize} plugin for font sizes both when you use one of the theme settings and when you use specific pixel values (e.g. {fontsize="72"} - for 72px text)
When used with the {fontsize} plugin, the type scale keys are:
x_small (12px) small (13px) medium (14px) base (16px) large (18px) x_large (20px) 2x_large (24px) 3x_large (30px) 4x_large (36px)  
Flexbox
While we've used flexbox in some places in previous versions, 4.5 makes much wider use of it and also introduces a new family of classes. If you aren't familiar with flexbox, I highly recommend this CSSTricks article for a primer on it. Essentially, instead of positioning elements using floats/clears/etc., flexbox treats the container as a flexible box with properties for controlling how elements inside of it as laid out.
4.5 has a number of new classes that are essentially just convenience for the usual CSS rules.
ipsFlex (sets element to display: flex) ipsFlex-ai:start, ipsFlex-ai:center, ipsFlex-ai:end, ipsFlex-ai:stretch (ai - values for align-items property) ipsFlex-as:start, ipsFlex-as:center, ipsFlex-as:end, ipsFlex-as:stretch (as - values for align-self property) ipsFlex-jc:start, ipsFlex-jc:center, ipsFlex-jc:end, ipsFlex-jc:around, ipsFlex-jc:between (jc - values for justify-content property) ipsFlex-fd:column, ipsFlex-fd:row, ipsFlex-fd:column-reverse, ipsFlex-fd:row-reverse (fd - values for flex-direction property) ipsFlex-fw:wrap, ipsFlex-fw:nowrap, ipsFlex-fw:wrap-reverse (fw - values for flex-wrap property) ipsFlex-flex:10 - sets flex-grow: 1 and flex-shrink: 0 ipsFlex-flex:11 - sets flex-grow: 1 and flex-shrink: 1 ipsFlex-flex:01 - sets flex-grow: 0 and flex-shrink: 1 ipsFlex-flex:00 - sets flex-grow: 0 and flex-shrink: 0  
All of these classes have md and sm prefixed versions too, and this opens up the possibility of having different layouts on different device sizes in a way that's much easier than the hoops you'd have to jump through before. For example, to create some elements that show as a row on desktop but collapse to a column on mobile, you'd just apply ipsFlex ipsFlex-fd:row sm:ipsFlex-fd:column. The sm:ipsFlex-fd:column class overrules the ipsFlex-fd:row class on mobile, adjusting the layout. (Note: flex-direction: row is the CSS default direction anyway, so you can actually leave out ipsFlex-fd:row - it's implicit. I included it in the example for clarity.)
 
Padding/margin
We've added new spacing classes for padding and margin, to allow for atomic classnames, device prefixes and modifiers.
ipsPad, ipsPad_double, ipsPad_half, and all of the ipsSpacer_* classes are now deprecated. You'll still see them in our templates and they'll still work in yours, but don't use them in any new work - you should use the updated classes below.
The padding classes are now named ipsPadding:
ipsPadding, ipsPadding:none, ipsPadding:half, ipsPadding:double - apply padding to all four sides ipsPadding_vertical, ipsPadding_vertical:none, ipsPadding_vertical:half, ipsPadding_vertical:double - apply padding to top and bottom ipsPadding_horizontal, ipsPadding_horizontal:none, ipsPadding_horizontal:half, ipsPadding_horizontal:double - apply padding to left and right ipsPadding_left, ipsPadding_left:none, ipsPadding_left:half, ipsPadding_left:double - apply padding to the left side (RTL aware) ipsPadding_right, ipsPadding_right:none, ipsPadding_right:half, ipsPadding_right:double - apply padding to the right side (RTL aware) ipsPadding_top, ipsPadding_top:none, ipsPadding_top:half, ipsPadding_top:double - apply padding to the top side ipsPadding_bottom, ipsPadding_bottom:none, ipsPadding_bottom:half, ipsPadding_bottom:double - apply padding to the bottom side These classes have md and sm prefixed versions too, allowing you to apply different padding depending on the device size.
One side note here: with the old padding classes, padding was simply halved on phones with no opt-out. That's not the case with the new family - if you want half-padding on mobile on an element, you should apply sm:ipsPadding:half in addition to the normal ipsPadding class, for example. This gives you much more control than you previously had.
Margins follow basically an identical pattern to padding, with the same variation of classes, except the name is ipsMargin.
 
Gaps
Another new family of classes is ipsGap. These classes are used when you want spacing between elements. In the past, you'd have to use :last-child or :first-child to exclude an element, or loop over the elements in the template to leave off a class. If elements wrapped to a new line, putting spacing between the lines was tricky too.
ipsGap solves this by applying even spacing between elements, then applying a negative margin on the whole container to bring it back to the starting position.
The classname is followed by a modifier, which is a number from our spacing scale, e.g. 1 is 4px spacing, 2 is 8px spacing and so on.
ipsGap:1 (1-5 available) - applies both horizontal and vertical spacing around each element in the container ipsGap_row:1 (1-5 available, as well as 0 to remove) - applies vertical spacing on each element in the container Notice ipsGap_row also supports the :0 modifier. This allows you to have horizontal-only spacing - simply apply ipsGap:1 ipsGap_row:0.
Be aware that using both ipsMargin (or custom styles that apply a margin) and ipsGap on the same element can cause issues. You may want add a wrapper element to handle your margin in this situation.
 
Borders
We've also added a class family to add light grey 1px borders to elements - used commonly as dividers between some parts of the page.
ipsBorder - apply border to all sides ipsBorder:none - remove border from all sides ipsBorder_vertical - apply border to top and bottom ipsBorder_horizontal - apply border to left and right ipsBorder_top, ipsBorder_bottom, ipsBorder_left, ipsBorder_right - apply border to a particular side These classes have md and sm prefixed versions too, to control borders shown on each device size. This is particularly useful if you apply a border to a flex child which is in a row on desktop but a column on desktop, for example - you will be able to easily control which side the border appears on once collapsed.
 
"Pull" class
To better display content areas on mobile, a class named ipsResponsive_pull has been added which 'pulls' a box on the left and right sides on small devices. It's intended to be used on boxes (normally with the ipsBox class) that you want to take up the whole screen width on mobile, allowing better usage of the available screen space.
 
Template changes
We've worked to keep template changes as minimal as possible, but in an update the size of 4.5 there are still quite a number of changes. Whether these impact you will depend on if you've modified the template (for theme designers) or rely on a particular selector for theme hooks (for developers).
One area that has received fairly big changes is the post/comment templates. We have redesigned the headers and footers of these templates and moved some elements into a separate parent element on mobile devices.
As usual, full template changes will be available once we've released betas.

The underline is being added because by default we underline all links in content created within the editor, and so when you wrap the review box in a link, all text inside it becomes underlined. This is the expected behavior even though it doesn't look 'correct' in this specific instance.
You could exclude the review widget by adding something like this to your custom.css file:
.grw-net-widget, .grw-net-widget * { text-decoration: none !important; }  

The underline is being added because by default we underline all links in content created within the editor, and so when you wrap the review box in a link, all text inside it becomes underlined. This is the expected behavior even though it doesn't look 'correct' in this specific instance.
You could exclude the review widget by adding something like this to your custom.css file:
.grw-net-widget, .grw-net-widget * { text-decoration: none !important; }  

The underline is being added because by default we underline all links in content created within the editor, and so when you wrap the review box in a link, all text inside it becomes underlined. This is the expected behavior even though it doesn't look 'correct' in this specific instance.
You could exclude the review widget by adding something like this to your custom.css file:
.grw-net-widget, .grw-net-widget * { text-decoration: none !important; }  

The user experience of the accordion menu on the notification settings page is really bad in my opinion. Whenever you click on one of the settings to expand the menu, it feels like it's lagging for a short period of time. I know that there is a loading indicator on the right side of the menu, but you can barely notice it because the menu expands too fast. (Which is good in theory but feels weird in practice)
I think this could be improved by expanding the menu and show the loading state on the already expanded accordion menu. Maybe even show a skeleton loading state for the notification settings?
 

At some point we will be making the GraphQL API available for general-purpose use, and that would likely suit your usecase better. Keep an eye out for news of that in due course.

At some point we will be making the GraphQL API available for general-purpose use, and that would likely suit your usecase better. Keep an eye out for news of that in due course.

.ipsApp_front is also a class on the body element, so you need to remove the space (I've also moved that class to the front of the selector for clarity):
body.ipsApp_front[data-pageapp="forums"][data-pagemodule="forums"][data-pagecontroller="index"] .ipsBreadcrumb { display: none !important; } An alternative way is using the data-pagelocation attribute in the same way you've used the others, since that tells you whether it's 'front' or 'admin':
body[data-pagelocation="front"][data-pageapp="forums"][data-pagemodule="forums"][data-pagecontroller="index"] .ipsBreadcrumb { display: none !important; } Either of those approaches should work 🙂 

Yep - and that's where we (or at the very least, I) see the future of Pages - but those apps also have huge open-source communities or a large development team focusing only on that product. It's not something we can click our fingers and bring to life, but it is in our minds.
Speaking personally, I have a pretty clear picture of what Pages could be and what would set it apart from the other visual page builders out there, but for the time being that'll stay internal until a definite roadmap is planned out.

font-display: swap won't really give any benefit to that particular font since it's used for icons - there would be no system font it can use temporarily. We can certainly experiment with font-display: block and see if that would improve how lighthouse sees the page though.

font-display: swap won't really give any benefit to that particular font since it's used for icons - there would be no system font it can use temporarily. We can certainly experiment with font-display: block and see if that would improve how lighthouse sees the page though.

It isn't currently high on our priority list. It's called in a third-party library we use.

All of this is on our radar - it's obviously not trivial though. Trust me, I probably use Pages more than anyone else, so I'm certainly aware of the pain points and potential future for it 🙂 

All of this is on our radar - it's obviously not trivial though. Trust me, I probably use Pages more than anyone else, so I'm certainly aware of the pain points and potential future for it 🙂 

Yep - and that's where we (or at the very least, I) see the future of Pages - but those apps also have huge open-source communities or a large development team focusing only on that product. It's not something we can click our fingers and bring to life, but it is in our minds.
Speaking personally, I have a pretty clear picture of what Pages could be and what would set it apart from the other visual page builders out there, but for the time being that'll stay internal until a definite roadmap is planned out.