yes dark theme's better if the screen brightness level stays the same

That is entirely incorrect.

There are two competing factors:

1) Dark text on a light backround produces less eyestrain

2) A screen that is of comparable brightness to surroundings produces less eyestrain

If you're using your computer during the day at moderate/normal brightness, then light mode absolutely produces less eyestrain than dark mode.

If you're using your computer at night turned up to maximum brightness, then light mode absolutely produces more eyestrain than dark mode.

At night, you should either use light mode but with brightness turned down significantly, or dark mode at moderate brightness.