Warning: Stylish has turned into malware!
Recommend you use one of the many alternate extensions, such as Stylus.
Stylus and Stylish (and others) are multi-browser extensions that customize the appearance of web-pages that you browse.
That is, they let you "Theme" or "Skin" pages you visit by allowing you to overwrite CSS styles. This is especially useful for fixing the horrible font-sizes and low contrast text that many sites inflict upon their users.
For more information, see:
- the Stylish add-on page (Firefox)
- the "Using Stylish" page at userstyles.org
- the Stylish extension for Google Chrome.
For ready-made scripts/themes, see userstyles.org.