- Within `extensions/Theme` there's a file called `extension.json`. You need to find the `ResourceModules` bit and add the theme there. For example mine looks like this:
- NB: I only have one theme set-up though! Each theme you want to use needs it's own 'module'.
### Extensionless Alternative
If you don't want to use a theme you can add the content of the stylesheet to MediaWiki:Common.css page of your wiki. To edit this page you need to be in the 'interface administrator' group. You can sheck this on the Special:ListUsers page.
The downside of this method is that it will be applied to every skin.