Thread:Project:Support desk/Updating CSS

MediaWiki version 1.15.2 PHP 5.3.5 (apache2handler) MySQL 5.1.54 site is www.climatewiki.org

I am new to MediaWiki but have to make some changes to the CSS pretty quickly -- our logo is missing! The person who managed our site beforehand is no longer accessible to us and did no training. Don't really know where to begin. I know CSS well, but how do I find the right stylesheet/skin and get it uploaded to our server? Or is there a "special page" on which I can access and change the skin? Didn't see on thee ...