I’ve been using MediaWiki for ages for technical documentation, and I always forget how to implement custom CSS in both screen and print, so I’m adding this as a permanent reminder for myself.
To edit the CSS for the site (screen version), navigate to “http://YOURMEDIAWIKISITE/index.php?title=MediaWiki:Common.css” - and edit the CSS declared on that page
To edit the CSS for printing, navigate to “http://YOURMEDIAWIKISITE/index.php?title=MediaWiki:Print.css”
Easy!