Stylish changes colors, spacing, fonts, and the arrangement of individual elements including advertising banners. On the developer page you can find a list of ready-made script files for websites, which you can load into the extension. If you've got any CSS skills, you can even create your own templates.
Overall, Stylish isn't exactly the most essential Firefox extension out there, but if you're really bored of the look of Firefox or can't stand the sight of a particular website, it´s worth a try.
- Updated to support Firefox 4.0b2