They’re supposed to reduce eye strain, but personally I’ve never experienced eye strain from any colour scheme, and suspect it has more to do with the eyesight of the user than the colours involved.
Staring into a bright screen in a poorly lit room is eye strain. The pupil is dilated because the room is almost dark so more of the light from the bright screen will hit the retina.
Blacking out the screen in those conditions reduces that strain. But you still have to maintain a good brightness and contrast between the elements otherwise you trade one issue for another.
There's no one size fits all unfortunately. Which is why it's great when developers allow for customization.
> Staring into a bright screen in a poorly lit room is eye strain.
That's a really good point.
If I have to use a computer or phone under poor lighting, I turn down the display brightness to match.
Otherwise, I prefer to be in a space where I can read something written on paper. And then I adjust the brightness to match that - not cranked up too bright like I see sometimes.
Personally I can work 16 hours with white theme but even 1-2 hours with strictly dark theme is borderline unbearable. I tried it multiple times due to posts advertising it's lower eye strain. I'm 30 and myopic, -4 in both eyes, using PCs for decades.
Opening a white window is like a full headlight in my eyes. Many scientific studies said that blue led (and then white RGB pixels) deteriorate the sight over time, by destructing photosensors in the eyes and they never recover. I would encourage black themes in all apps.
Apparently my direct experience gets downvotes. I literally use light themed apps in low light conditions every day and have never experienced any discomfort that could be called eye strain. For at least some set of people it simply does not exist. I therefore put the variance down to the individual’s vision.
I think it would largely depend on the display technology. I know for some technologies a black pixel uses nearly no energy, but for others, all pixels are equally 'lit' by the same backlight.
For OLED displays, black uses (almost?) no energy. For LCD displays, black uses more energy, since the pixel has to be energized to block out the backlight. The difference is negligible, though, since the backlight uses far more.
Blacking out the screen in those conditions reduces that strain. But you still have to maintain a good brightness and contrast between the elements otherwise you trade one issue for another.
There's no one size fits all unfortunately. Which is why it's great when developers allow for customization.