- On the url bar type about:config and enter.
- Search for toolkit.legacyUserProfileCustomizations.stylesheets and enable it.
- Then go to about:profiles
- You will find a root directory. Go there.
- Create a folder named chrome
- Open terminal and run git clone https://github.com/siyad23/Minimalistic-Firefox-Theme.git/.
- Restart firefox.
If you want to change the wallpaper, just simply replace the wall.jpg with any of your favourite wallpaper and name it wall.jpg assuming you don't want to edit css file.