Stylem - A user style manager for Pale Moon, Basilisk, Interlink, SeaMonkey, and other Mozilla-based software. Install styles from userstyles.org to change how web pages and the application look.
Stylebot - Change the appearance of websites instantly. Preview and install styles created by other users on stylebot.me
Wikiwand - A beautiful new interface to Wikipedia
FreeStylerWs - Styles and skins for all popular sites in the web
Amino Editor - Amino is a Chrome browser extension for customizing web page presentation with user CSS.
Stylus - EXPRESSIVE, DYNAMIC, ROBUST CSS