Most vanilla browsers lets you add your own user style into cascade. In Firefox it is userContent.css and userChrome.css [1] (loaded at startup), in Google Chrome custom.css [2](this file is 'live'). IE [3] and Opera [4] have something like this as well.

[1] http://www-archive.mozilla.org/unix/customizing.html [2] http://code.google.com/p/chromium/issues/detail?id=2393 [3] http://windows.microsoft.com/en-US/windows7/Use-your-own-sty... [4] http://www.opera.com/docs/usercss/#user-mode