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
Tampermonkey - Greasemonkey compatible script manager.
Dark Reader - Reduce eye strain in your browser with this extension that provides a dark theme for browsing.
Violentmonkey - Violentmonkey is a userscript manager to support running userscripts in web pages.
Stylus - User Styles Manager - Stylus is a userstyles editor and manager based on the source code of Stylish version 1.5.2.