Stylus for Firefox is a powerful, open-source user styles manager that allows you to redesign and theme any website using custom CSS. It was created as a privacy-friendly, tracking-free alternative to the older “Stylish” extension after that project was sold to an analytics company. Key Features
Complete UI Overhauls: You can change colors, modify layouts, widen content areas (like making GitHub wider), hide distracting sidebars on YouTube, or force dark modes on websites that lack them.
Instant Live Previews: Modifications are injected into web pages in real-time, allowing you to see visual changes instantly without reloading the page.
Granular URL Targeting: Themes can be configured globally across all websites, or restricted to specific URLs, entire domains, or complex patterns via regular expressions.
Built-in Advanced Editor: For custom creation, it features a full code editor supporting CSS, LESS, and Stylus syntax, integrated with up-to-date linters, autocomplete, and optional custom keymaps like Vim or Emacs.
Cloud Synchronization: You can effortlessly back up and sync your entire repository of custom styles across devices using Dropbox, Google Drive, OneDrive, or WebDAV. How to Install Existing Themes
You don’t need any coding knowledge to use Stylus, as a massive library of community-made themes is available.
Install the extension from the official Firefox Add-ons Store. Navigate to the website you want to change (e.g., YouTube).
Click the Stylus icon in your browser toolbar and select Find styles.
Browse or filter the community themes from external repositories like UserStyles.world or the openstyles/stylus GitHub Wiki. Click a theme to install it instantly.