Honestly, the mania for dark mode baffles me.
Dark mode isn't for everyone, but it does seem to suit a lot of people - enough that 3rd party themes pop up for popular websites and desktop apps.
Like at a bus stop in the evening, long distance buses, during flights, while one has difficulties to sleep next to a partner.
I struggle to see how stopping to read in dark environments altogether is a serious proposal to the problem.
One way to reduce the amount of blue light coming into the eye, it seems to me, is to add red light, which causes the pupils to contract, which makes it so that less of the blue light outside the eye will enter the eye.
I get the impression that the light coming from a black pixel has a lot of blue in it, and that turning on the red sub-pixel would decrease the amount of blue light entering the eye because the pupil-contracting effect I just described (even though it would not decrease the amount of blue light outside the eye).
My point is that using f.lux to dial down the color temperature as low as it will go (1200 K on my Mac after bedtime, giving the screen a pronounced reddish-orange hue) will result in less blue light entering the eye than making most of the pixels black would result in.
(Turning on a red light bulb would have the same effect, but f.lux is easier.)
A better way to avoid blue light would be to switch to an OLED display or to have a backlight that can be switched to producing only red light, but among the no-purchase-cost options, if the only displays available are LCDs with standard white backlight, making the screen mostly red seems to work better than making the screen mostly black.
(Neither is particularly good however if the goal is to avoid the effects of blue light on the brain.)
I don't know which colors are most effective at causing the pupils to contract. I don't have any hard data. But I'm sensitive enough to the effects of blue light on my brain at night that I believe it worthwhile to post here my subjective impressions of the relative effectiveness of f.lux adjusted (via its preferences pane) as red as it will go and a mostly black screen, which I achieved by making most things white, including a solid-white desktop, then "inverting the colors" using the keyboard shortcut control-option-command-8.
(It is possible to have both of the "interventions" or "settings" described above in effect at the same time, so I will add that once f.lux is as red as it will go, "inverting the colors" does not produce any additional benefit as far as I can tell.)