When I visit a dark theme website, all is good for maybe half a minute or so. As you say, less light hitting your eye is somewhat pleasant. But then, somehow, the eye strain starts. My eyes seems to defocus, and it becomes harder to make out letters. I have to squint, and after maybe a minute that no longer helps. If I don't stop reading then, but keep going for a few more minutes it gets progressively harder. Then when I look away, I am blind.
No kidding, can't see a thing, just a grey blur everywhere. Takes several minutes to get back to normal.
I have no idea how common this is. I asked an optometrist about this once, and she looked at me like I was from Mars.
Dark themes are not the solution for everyone, and default dark themes everywhere is my personal nightmare.