The dark portions of the screen block the backlight, so the darker it is, the less light gets to your eyes.

In a low luminosity environment, you will get better contrast with less emission if you have most of your screen dark and slightly more backlight than with most of it lit and less backlight. You will want to adjust the light anyway, but the dark mode is theoretically a gain.

But all of that is moot anyway, because the web doesn't work in dark mode. So if you try to go with it you will keep being exposed to high brightness at random, what is much worse than any alternative.