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firefox-dark-userContent.css
css to compliment firefox's colors option for dark mode on all websites
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FirefoxCSS-Store
A collection site of Firefox userchrome themes, mostly from FirefoxCSS Reddit community.
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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
i've been working on my own userContent.css for a few days now and it might be able to help you get started, it's hosted on github if you are interested
people here usually talk about the browser itself which mainly has to do with userChrome.css if that's you goal look here for examples and inspirations