superUserContent is described as 'Dynamically load stylesheets in Firefox from plaintext files in your profile' and is an app in the Web Browsers category. There are eight alternatives to superUserContent for a variety of platforms, including Google Chrome, Windows, Linux, Online / Web-based and Mac. The best alternative is Dark Reader, which is both free and Open Source. Other great apps like superUserContent are Stylus (Free, Open Source), Stylebot (Free, Open Source), Amino: Live CSS Editor (Free) and Stylem (Free, Open Source).
This eye-care extension enables night mode creating dark themes for websites on the fly. Dark Reader inverts bright colors making them high contrast and easy to read at night. You can adjust brightness, contrast, sepia filter, dark mode, font settings and ignore-list.
Make visual changes to any website with CSS and see changes in real time. Sync, edit, and export your custom style sheets. - Customize websites with CSS by creating your own user style sheets. Customize page appearance, fix rendering bugs, make the web what you want it to be.