Created and Functional on Windows 11 with version 136.0 of Firefox.
This theme uses stylesheets from the MrOtherGuy firefox-csshacks repository.
You can download the files by clicking HERE.
In the Firefox Address Bar, navigate to about:config and accept the risks
Search for toolkit.legacyUserProfileCustomizations.stylesheets and toggle it to true (by double clicking on it)
In the Firefox Address Bar, navigate to about:support, Search for Profile Folder and click on the Open Folder button
Create a new folder and name it chrome (with lowercase)
Unzip and paste the downloaded files inside the chrome folder
Restart Firefox, and enjoy your new theme!