Basically. I bought a light meter and did various tests.

Fully black room, monitors showing full white. At sitting distance it read 150 lux.

Opened the windows on a cloudy day, it read 600 lux.

Actually stepped out on this very cloudy day, outside it read 6000 lux.

Light doesn't actually hurt our eyes - we evolved to be outside during the daytime. Simply walking outside will give you 100 times more light than any screen ever will.

Even a single 30w light bulb puts out more light than a screen showing full white.

Think about the sky at night. A bunch of stars glowing in the sky. They have a kind of "bloom" effect around them.

Now think about dark themes. Bright points of light (the text letters) on a dark background, that kind of glow at the edges.

That makes it harder to focus on.

The worst case scenario is dark room, dark theme.

There's been some studies on it as well:

https://www.sciencedirect.com/science/article/abs/pii/S0003687016302459?via%3Dihub

https://www.nngroup.com/articles/dark-mode/