Changing css inside custom admin page?

I'm really intrigued by the TwentyTen themes ability to allow the user to load their own logos/backgrounds/headers onto their blogs theme without touching any css or html. Instead, you can go under appearence and there is a custom tab where you can change that.

I was wondering if anybody has any good reads on doing stuff like that, but also goes into more depth. I want to be able to have users change even the page width, css link colors, etc. I know woothemes has a framework like that, but I want to start my own and don't really know where to start. Help would be appreciated.