Windows, Mac, and Linux: Firefox extension Stylish lets you install user-defined CSS templates, allowing you to tweak any web site of your choosing to your heart’s content.
If you’re unfamiliar with how web sites are styled, CSS, or cascading style sheets, are the backbone that defines what HTML elements go where and how everything will look on a web page. If you’re still scratching your head, that’s no problem – web site Userstyles.org has a collection of user-created styles for several web sites that you can search through. Tweaks made with Stylish can be as simple or extreme as you like – for example, check out this Google Dark script designed to make Google search easier on the eyes. Thanks Josh!