User blog comment:Lancer1289/Wikia's New Skin, the countdown begins/@comment-1074875-20100928213440/@comment-2088100-20100928221032

For instance, right now on Monaco, the site's look is governed (in part) by MediaWiki:Monaco.css, MediaWiki:Common.css, and MediaWiki:Common.js. These download and apply to everyone who visits.

Your own personal Special:MyPage/monaco.css and Special:MyPage/monaco.js apply only to you, and will only be downloaded for you. For instance, I currently have a rule in User:Dammej/monaco.css that limits the width of the content space to 660px, so I can see just how small the space will be in articles once the switch to the new look is complete.