Board Thread:Support Requests - Designing Your Wiki/@comment-11363587-20130804220023/@comment-1330470-20130805022240

Well, best to explain the CSS pages.
 * Mediawiki:Wikia.css affects only users using the default Oasis/Wikia/"New Wikia Look" skin (99% of users)
 * Mediawiki:Monobook.css affects only users using the Monobook skin.
 * Mediawiki:Common.css is "common" to both Wikia and Monobook skins... theoretically. You see, Wikia specifically disables it for the Wikia skin (making it "common" to exactly one skin: Monobook), and while it's useful for large wikis to have a page that's used by both skins, it also requires special import code to overcome Wikia's cripple. Because of this technicality, you might be best off using Wikia.css.