preview:

new tab

new private tab

guide:

1. go to about:config in your browser
2. change toolkit.legacyUserProfileCustomizations.stylesheets to true
3. create dir named chrome and put theme files there
   • path on windows: C:\\Users\\username\\AppData\\Roaming\\Mozilla\\Firefox\\Profiles\\*release\\chrome
   • path on linux(firefox native): ~/.mozilla/firefox/*release/chrome
   • path on linux(snap): ~/snap/firefox/common/.mozilla/firefox/*release/chrome