Board Thread:Support Requests - Designing Your Wiki/@comment-4192163-20130320203424/@comment-3335785-20130321142808

Ah, I see the problem.

Despite the somewhat misleading name MediaWiki:Common.css isn't actually imported to both skins (this is something Wikia decided to do and I have no idea why). Monchoman45 keeps a blog of useful tips, this is the one that'll fix your issue.