Board Thread:Support Requests - Designing Your Wiki/@comment-31702336-20170413120644/@comment-168424-20170414174728

Typically, MediaWiki:Wikia.css is for the default "Oasis" skin (called "Wikia" in Preferences), so that is likely the one you want to think about.

See Help:Community CSS and JS for load order info about MediaWiki:Wikia.css and MediaWiki:Common.css.