Hi there, thank you for the feedback! The add-on should work right after installing it once. You might need to reload your tab with YouTube or open a new one in order for the script to load in. Your OS has no influence on how well the add-on works.

If you are interested in helping me improve the add-on, you are very welcome to contact me so I can ensure it works for you and possibly other people having your issue. Sadly, writing that it "works but without any real results" does not give me anything to work with whatsoever.

You can contact me by opening an issue on GitHub:
https://github.com/Joooohannes/Less-Mixes-for-YouTube

or directly via e-mail:
johannes.ji1702@gmail.com