To use this theme on the web, you gust have to install this extension. To do so, go to the Chrome web store then click on install. That's it!

Personally, I use the YouTube Music app (unofficial). To us the theme in the app. You just clone the repo (git clone https://github.com/Angus-Paillaugue/YT-Music-Nord-Theme), then compile the scss file by running ./compile.sh, after compiling, move the ./prod-css/yt-music.css file wherever you want to store it (I put it in the app config directory : /.config/YouTube Music). Then import the theme via Options -> Visual Tweaks -> Theme -> Import custom CSS file : Then select the css file from the cloned repository.

To modify this extension to suit you. You can clone the repository (git clone https://github.com/Angus-Paillaugue/YT-Music-Nord-Theme.git). The color palette is located the the nord.scss file. The actual css for the pages is in ./dev-css/. There are multiple scss files (one fcontent_scriptsor each page). To compile the files, there is a util bash file named compile.sh. Just run it and it will create the prod-css directory and compile all the files in dev-css and put their compiled version there. If you want to add a new custom page, don't forget to also add the entry in the manifest.json under content_scripts.