Stylish

With Stylish, you can completely customize the look of your favorite websites.

The extension provides your access to a large gallery of user-submitted themes and styles for various popular websites including Facebook, YouTube, Google, Twitter, Tumblr and more. You can pick a theme and the next time you visit that website, the chosen style will automatically get applied.

Internally, a style is nothing but a bunch of CSS rules that are auto-applied to the corresponding website. Thus, themes can make sites more readable, remove sidebar and other irrelevant content, apply background images and more.