Stylish

Stylish allows easy management of user styles. User styles empower your browsing experience by letting you fix ugly sites, customize the look of your browser or mail client, or just have fun. With an online repository at userstyles.org, you don’t even need to know how to write styles yourself; just a couple clicks and the chosen style is applied. Stylish is to CSS what Greasemonkey is to JavaScript, and unlike other methods of using user styles, most styles take effect immediately.
User guide: http://userstyles.org/stylish/help
Bug reports and feature requests: http://userstyles.org/stylish/discussion