Board Thread:General Discussion/@comment-1416490-20131211031151/@comment-16101931-20131213035309

What exactly are you wanting to do? It shouldn't violate Wikia's Terms of Use agreement unless you do something to the ads or the Wikia header bar, or drastically change the way the site works. But for your personal CSS file, located here on Community Central (Special:MyPage/global.css), you can do anything you want.

If you want to make site-wide styles you can use MediaWiki:Wikia.css, MediaWiki:Monobook.css, or MediaWiki:Common.css (though the latter should only be used for styles which can affect both skins).

Just depends on what you want to do.