[Greasemonkey] Apply User Stylesheet (CSS) rules on a site-by-site basis

[Bill Donnelly]

There is also this one:

Stylish 0.3.2, by Jason Barnabe (np), released on May 4, 2006

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.

https://addons.mozilla.org/thunderbird/2108/