YouTube Design Preserver is a browser extension that allows you to choose between the old, dark or normal YouTube design and preserve it permanently!
https://addons.mozilla.org/en-US/firefox/addon/youtube-design-preserver/
https://addons.opera.com/en/extensions/details/youtube-design-preserver/
To build this project you have to have the following tools installed:
sed jq zip nodejs npm
Once you have installed all of these tools, you can build the project with these few steps:
npm install
npm run build
The resulting files are going be in the dist directory. Note that the chrome files will work with all chromium based browsers.
YouTube Design Preserver will not work in private browsing mode for Firefox versions older than 52.