I have the same Problem, I have test to fix this with my one Theme. But i can not change the right Color. My Work is now to 90% ok for me.
Use for v7 Firefox Color, Look in .xpi (zip) Sources.
https://addons.mozilla.org/de/developers/
with extra Graphic
rgba(227, 228, 230, 0.1) rgba(0, 0, 0, 1)
rgba(227, 228, 230, 1) x
x x
x
no extra Graphic
rgba(0, 0, 0, 0) rgba(0, 0, 0, 1)
rgba(227, 228, 230, 1) x
x x
x
Have only found this:
Us this Eye Friendly Theme from User dragoniccaster
Please give us the Default Theme from Firefox 88.0.1 back to Firefox 89.0 or as Extra Download