Right now I'm using this to set a different width for the forum app only:

body[data-pageapp=forums][data-pagelocation=front][data-pagemodule=forums][data-pagecontroller=index] .ipsLayout_container

The issue is that when I enter a forum or a topic, the pagecontroller changes changes to forum and topic respectively. Same when viewing an user profile, it changes to profile.

So... is there a way to specify the CSS for the whole forums app?

Couldn't you use only 

body[data-pageapp=forums]

as selector?

...right, thanks. Also, I don't want that CSS to affect the site when using a mobile device, any ideas?

body[data-pagecontroller="index"] .ipsLayout_container { max-width:.....}

Thanks, I found what I needed. I only wanted to modify the width of the forums app on desktop view only. This worked:

@media screen and (min-width: 980px) { 
  body[data-pageapp=forums] .ipsLayout_container 
  {
  	min-width: 1120px;
  	max-width: 1340px;
  }
}