Seriously, the best option is whatever matches the brightness of your screen to its surroundings. I read about this decades ago and it eliminated screen fatigue for me.

If switching to dark mode works for you, great. When I worked on a PC in a well-lit office all day, I would open a program with a white background, hold up a blank white piece of paper next to the screen, and adjust the screen brightness until it looked about the same as the paper. I did this once or twice a week because I was near a set of picture windows and I was affected by weather and the seasons, but in a room with more artificial light it would be "set and forget".

It seemed very dim at first, and several of my coworkers commented on it. It took a few days of resisting the urge to turn the brightness back up, but I got used to it and never went back.

My PC at home is currently set up in a partially shaded corner of a well-lit room, so I put a dim little light bar behind the screen to make the wall match the brightness of the screen and the rest of my desk/room.

First time I've ever heard of anyone actually using time-of-day-dependent themes

This is the way.
I come into the office early in the morning when its still night and use the dark theme. When the sun comes out, I switch to light. Monitor brightness should blend into the surrounding light. Eye strain otherwise.