It's just that editing things involving CSS (that aren't text in-line CSS tags like <table> <cell> and <big>) aren't allowed on ordinary webpages, perhaps because it could lead to website security violations. So it would make sense that I would not be able to anything involving the <style type="text/css"> tag
The only wiki-wide common.css is this one but you can't edit it. Maybe user specific common.css might do the trick but it might have unintended consequences such as accidentally making the whole wiki's text bigger, which actually happened.
There's also this comment regarding displaying fonts through css:
I'm not sure if that's taken to be a warning or discouragement or general comment...I'm pretty sure font tags don't work, so it needs to be CSS like that. Mind you, it's a very un-wiki way to do things. —Muke Tever | ✎ 18:33, 3 December 2007 (PST)