Board Thread:Support Requests - Designing Your Wiki/@comment-26368573-20121230230432/@comment-4674838-20121231000702

It's JavaScript, not CSS. There is one script here: http://community.wikia.com/wiki/MediaWiki:Snow.js that does that and you can use it if you like. It was previously used here on Community Central, but it was disabled because a number of users filed complaints about it slowing down the site.

If you're talking about a different wiki, it might be that they have their own snow script that's different from the one I just mentioned. I suggest going to the wiki where you saw this and asking one of the admins about it if you're interested in their script.