FreeStylerWs - Styles and skins for all popular sites in the web
Stylebot - Change the appearance of websites instantly. Preview and install styles created by other users on stylebot.me
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.
Amino Editor - Amino is a Chrome browser extension for customizing web page presentation with user CSS.
xStyle - A user styles manager for the Web
Dark Reader - Reduce eye strain in your browser with this extension that provides a dark theme for browsing.