In newer MediaWiki versions (1.35+), the process for adding custom CSS has changed from older versions.

There are now several options to use custom css styles for MediaWiki, below I want to show how to use your own custom stylesheet file by adding a new line in the LocalSettings.php configuration file which refers to your own created custom css file.

In your LocalSettings.php, add:

$wgHooks['BeforePageDisplay'][] = function( OutputPage $out, Skin $skin ) {
    $out->addStyle( '//example.com/path/to/your/styles.css' );
    return true;
};


In case you will not see the new styles applied immediately, we can append the following to our URL.

?action=purge

Links

MediaWiki
https://www.mediawiki.org/wiki/MediaWiki

Manual:LocalSettings.php
https://www.mediawiki.org/wiki/Manual:LocalSettings.php