The IPS Community Suite uses CKEditor to power its rich text editing capabilities. Often, when developing a theme for you own community or for distribution, you'll also want to style the editor to match.
CKEditor itself is a very large and complex project. It has many hundreds of CSS classes, and explaining how to style each part of the editor is beyond the scope of this guide. We recommend you check out CKEditor's website if you need more information.
That said, here's some tips to guide you.
Using custom.css to style the editor
Prior to IPS 4.1, we used CKEditor's older iframe mode. What this meant is that the entire text editor existed inside of an iframe on the page (although this was seamless and transparent to users). Since styles of a parent page do not apply to the contents of iframes, the only way to style the editor was to edit the CKEditor skin that we shipped with the product. This meant working outside of the IPS4 theme system, but more importantly it made distributing your changes more difficult because you'd also have to distribute your CKEditor skin.
In IPS 4.1 onwards, we use the newer div mode. Instead of using an iframe, the editor is built inside a div element right on the page. This is great news for themers, because it means the CSS styles you create within IPS4 will be inherited by CKEditor automatically.
So, to start styling the editor, you can simply open your custom.css file in your theme, and using a tool such as Google Chrome's Web Inspector (or the equivalent in your browser of choice), inspect the HTML that CKEditor generates and use that to develop your styling. When you save your custom.css file, you'll see it applies to the editor too.
Above: inspecting CKEditor's generated HTML with Web Inspector allows you to see which CSS styles (right) are being applied to each part of the editor, helping you identify which class names you should be using in your own CSS.
Or build a standalone CKEditor skin
If you intend to make more substantial changes to the editor, you may still want to consider developing it as an actual CKEditor skin instead. CKEditor has a very mature skin framework that can be used for advanced changes. Consult the CKEditor for more information on creating a skin.
If you go this route, you would export the CKEditor skin, and ship it with your IPS Community Suite theme. When an administrator installs your theme, they can install the CKEditor skin in the AdminCP too.
So, that seems quite straightforward - in almost all cases, simply edit the custom.css file you use in your theme, and you can customize CKEditor to match your theme!
But, there's gotchas...
There are exceptions, of course. Even in div-mode, CKEditor still generates some iframes. For example, when you click a dropdown menu in the editor (e.g. Font), CKEditor actually builds an iframe for the menu.
This introduces the same problems we discussed above, again!
Unfortunately, there is no simple answer here. As before, styles you build into your custom.css file won't apply to these special areas where CKEditor uses an iframe. For many theme designers, this won't be a huge problem - being able to edit the 95% of CKEditor available to custom.css will be sufficient. But if you really do need to style the contents of those iframes, the only option is to do it within CKEditor's own skin system (since it loads those CSS files within its iframe).
This isn't too problematic if you're only concerned with styling your own community. The CSS files CKEditor uses can be found in /applications/core/interface/ckeditor/ckeditor/skins/ips (if your theme uses the default CKEditor theme we provide). Edit the editor.css file in this directory to adjust the styles (this is a compressed CSS file, so add your own CSS at the end - don't edit existing CSS in the file!).
If you plan to distribute your IPS4 theme, however, and you need to style these areas of CKEditor that still exist in an iframe, you'll need to go back to using CKEditor's skin system, and distributing a CKEditor skin with your theme.