Board Thread:Support Requests - Designing Your Wiki/@comment-7505640-20130611103222/@comment-1158325-20130611122908

It can take some time to take effect; if it still doesn't work 5-10 minutes after you added it and you've cleared your cache, there might be an issue. Common.css is (well, is supposed to be) applied to all skins, while Wikia.css is only applied to the Oasis (default) skin.