Greasemonkey - Customize the way a web page displays or behaves, by using small bits of JavaScript.
Stylebot - Change the appearance of websites instantly. Preview and install styles created by other users on stylebot.me
Violentmonkey - Violentmonkey is a userscript manager to support running userscripts in web pages.
Dark Reader - Reduce eye strain in your browser with this extension that provides a dark theme for browsing.
GreaseKit - GreaseKit is a SIMBL plugin, that adds user scripting to Safari, Mailplane, Diet Pibb.
FreeStylerWs - Styles and skins for all popular sites in the web