Stylish

User styles are themes for web sites. User styles empower your browsing experience by letting you customize web sites. Take out irrelevant content, change colors, or completely redesign the entire site. You can even use user styles as themes on the interface of your browser, Thunderbird, and SeaMonkey themselves.

Stylish lets you easily manage user styles. Add, delete, enable, disable, and organize with a few clicks of a mouse, no code to edit, no obscure configuration to find. Stylish's companion website, userstyles.org, hosts tens of thousands of user styles made by other Stylish users that you can try.

For you technical types out there, think of it this way: Stylish and userstyles.org are to CSS as Greasemonkey and userscripts.org are to JavaScript.