Can I just paste that section right into Mediawiki:Monobook.css inside Mediawiki?
Yup, that should work. If it doesn't, you could take a look at other wikis*; that should give you a clue.
-- F.
* e.g. http://www.mediawiki.org/wiki/MediaWiki:Monobook.css and http://www.mediawiki.org/wiki/MediaWiki:Common.css