This is different — user CSS requires a login and can override a skin, but is separate from skins, which are installed on the back end similar to an extension.

This is different — user CSS requires a login and can override a skin, but is separate from skins, which are installed on the back end similar to an extension. Similarly, skins can have their own custom CSS in a similar namespace. Also, you can use the `useskin` URL parameter to preview one of those backend-installed skins without logging in first, while user CSS requires a login.