Board Thread:Support Requests - Designing Your Wiki/@comment-27612891-20170212030223/@comment-168424-20171120185528

Personal CSS should go in yourwiki.wikia.com/wiki/Special:MyPage/wikia.css (for default Oasis skin) or /common.css (for all skins). See Help:Personal CSS and JS.

Wiki-wide CSS should go in yourwiki.wikia.com/wiki/MediaWiki:Wikia.css (for default Oasis skin) or /MediaWiki:Common.css (for all skins), but may depend on how old your wiki is. See Help:Community CSS and JS.