User blog comment:Sannse/Your First Look at the New Wikia/@comment-3100721-20100821081800/@comment-417287-20100822004936

Fandy, there is no such thing as User:Foo/common.css or .js. The way it works is /skinname.css or /skinname.js, 'common' is not a skin name. common.(css|js) was a mw feature, they didn't implement that as user skin subpages. Wikia has global css/js pages though, which would work as common.(css|js), but affect every wiki.