Board Thread:Support Requests - Getting Technical/@comment-4839035-20130202235147/@comment-4674838-20130203005042

Ok. The way you'd want to set up your code then is like this:
 * 1) "MediaWiki:Wikia.css" contains all the code for the default way the wiki looks to everyone.
 * 2) You load an additional, separate stylesheet for autoconfirmed (non-anon) users that overrides the default look.
 * 3) *This can be done by putting the CSS in MediaWiki:Group-autoconfirmed.css

There is also a MediaWiki:Group-sysop.css that will apply CSS rules to only admins.

But there is no such thing as "MediaWiki:Group-anon" etc. for logged-out users -- that's what "MediaWiki:Wikia.css" is for.