Actually, more like https://addons.mozilla.org/en-US/firefox/addon/2108

Stylish is an extension made specifically for applying external CSS to sites. Not sure, it might even be based on Greasemonkey, actually... Anyway, it's a good starting place, and I've seen a few very impressive things done with it (mostly to Gmail). If you do end up making any themes using either Stylish or Greasemonkey, please let us know.