After a few issues, I've noticed that the classnames are kinda capricious: it may changes from an user to another. BUT ! There is like 80% chance that this would works in your side.
The things that would interest you are the index.html file and the styles/ folder.
In this, you can see a list of input and labels. The type attribute for the input MUST be checkbox. The id attribute is the stylesheet that would be activated if the checkbox is checked. That's all.
Here, you have a css file for each option in the Chrome Extension. Name the file it like the id attribute in the input of the index.html file.
This is a Chrome Extension, so after you cloned this repo, go to the url chrome://extensions/, activate Developer Mode, and in "Load unpacked extension", open this repository.