Board Thread:Support Requests - Designing Your Wiki/@comment-4407814-20131220115207/@comment-4018164-20131220132310

You can try and add it via [ MediaWiki:Wikia.css?action=edit&useskin=monobook], though i think that the intent of disabling it from Special:CSS was to reduce the requests for resources like CSS files.

It seems like in my wiki MediaWiki:Common.css is now imported by default. I think that perhps Wikia listed wikis that import it and made MediaWiki:Common.css imported by default, maybe you should ask them via Special:Contact/general for more details about this, or use monobook to add it for now.