I think the issue is that people are adjusting what they see on screen instead of adjusting their environment (i.e. room lighting), even when they are totally able to do it.
A dark mode could make it bearable to work in poor light conditions but won't improve on the fact that it's already a non-optimal environment.
I wish my son would just turn the lights on in his room when he's gaming. He's just sitting there in the dark.
Mind you, I do the same when watching TV. Some shows and movies these days are so dark you simply have to turn off the lights and close the curtains to see what's going on.
A dark mode could make it bearable to work in poor light conditions but won't improve on the fact that it's already a non-optimal environment.