Default Theme from FF 88.0.1 제작자: Gantnerer

Please give us the Default Theme from Firefox 88.0.1 back to Firefox 89.0

Or us this Eye Friendly Theme from User dragoniccaster
https://addons.mozilla.org/de/firefox/addon/classic-gray-theme
You can use my Work or be it for Firefox 89.0 and above.

https://discourse.mozilla.org/t/changes-to-themeable-areas-of-firefox-in-version-89/81075/3
Delian_Tashev say: (Fixes that problem)
about:config
browser.proton.enabled - set to false