Board Thread:Support Requests - Designing Your Wiki/@comment-36535642-20200415111453/@comment-26923519-20200416083136

Ah, that's because you added it in the wiki's Common.css page. In the end, it doesn't really matter which page you add, because nowadays, they're practically the same.

If you wanna know what was the difference, Wikia.css is for the Oasis skin (default desktop skin), while Common.css was for both Oasis and Monobook, a desktop skin that was deprecated on May 25, 2018.