Hmm. It should be working. Could you give me some more details on what exactly isn't working?
What did you try, what did you expect to happen, what actually happened?
What operating system are you using?
Feel free to open an issue here: https://github.com/GuyXY/YouTube-Design-Preserver/issues
or send me an email: guyxy@mail.ru

Edit: For those who are curious what the problem was/is, it's a conflict between this extension and https://addons.mozilla.org/en-US/firefox/addon/policeman/
If you still want to use this extension and you are currently using the Policeman extension, I would recommend you to check out https://addons.mozilla.org/en-US/firefox/addon/umatrix/ as an alternative and see if it fits your needs.