Used dark (not black) themes everywhere for 8 years. My eyesight is still good according to my annual physical, but recently I’ve noticed that I have a hard time reading text written on a dark background. It is slightly blurred, especially when there is no light in the room.
Somewhere I still use dark themes, but I always try to switch to light mode if things look okay with code highlighting or smth.
Physical? As in a medical exam with a doctor?
If so you should really have a check up with an eye doctor, there are lots of eye health tests that you should regularly get beyond checking that you can read a chart at a distance.