As usual you have several options to add custom CSS to your MediaWiki installation (see also the manual pages on mediawiki.org):
- via Mediawiki:Common.css: be aware that it is not used for the login and preferences pages (you can, hower, allow it if you don't care for the security implications)
- via Mediawiki:Tweeki.css: more specifically, this CSS will only be loaded when you're using Tweeki as a skin. Same exceptions as above!
- via your own extension
$wgResourceModules['xxx'] = [ 'styles' => 'styles.css', 'localBasePath' => '.', 'remoteExtPath' => '', 'position' => 'top' ];