Better even than f.lux was Nocturne (https://github.com/strider72/blacktree-nocturne), though it hasn’t worked in years. (Maybe a decade? I can’t even find screenshots.)

Nocturne would let you invert your entire screen, then shift it to monochrome (ideally red). Obviously useless for gaming or color work, but I’ve not run across anything since that’s so great for late night text work across the whole OS.

I've never seen Nocturne but on Windows 10 inverted colors+night shift works well (IIRC has an option for grayscale, too). On Linux you can achieve the same effect with xca+redshift (not sure about grayscale).

I have switched all programs to darkmode where possible. If not possible I don't use it.

f.lux can do invert and redshift though: shift+alt+end