Focusing your eyes (and to a lesser extent, adjusting your iris to changing brightness) is like making a fist.
In a day’s reading, that’s happening about 30,000 times. Bit of a strain. What can we do to cut down that number or lessen the amount of muscle-force needed? Keep your irises closed down. Happens in photography all the time. Small iris-openings means a wider depth-of-field. Focused for 1m, with a small iris setting, objects at .5 and 1.5m might appear sharply focused. A really wide-open iris setting, and suddenly only objects at .9 to 1.1m are in focus.
Your eye sweeps the page/screen as you read a line. It’s unlikely that surface is perfectly curved so every letter is exactly the same distance from your eye, so a little focusing might be needed from the left, through the middle, to the right of the line. (That’s where they came up with the 30k estimate.)
So if we could pick an ideal, minimal iris opening to minimize that re-focusing during the scan, wouldn’t it be easier on our eyes?
And how do we get that minimal iris opening? With a brighter average scene. Light mode. Or perhaps “light-ish mode.” As many people have pointed out, extremes aren’t our friends. But we need contrast to read, we just don’t need it to be at 11 all the time.
YMMV. @HousePanther, you might need glasses. Strain=bad, but cyclical strain vs steady-state strain might be worse. You do you, I’m sticking with white or light backgrounds.