Well I went full mental and simply bruteforced it [1]. I'm doing it for page content as well with userstyle.

[1] https://gist.github.com/myfonj/f5415dd0580663a82ea18407ef2ee... This file starts with instructions how to make it work; I'm on Developer edition, Windows, dark theme. Same instruction wit illustration can be seen in Twitter thread [2] [2] https://twitter.com/myfonj/status/1387584962354982912