Apparently light-on-dark is terrible if the person has astigmatism: the edges get very blurry and doubled. There was an article on HN about this, specifically about keynote slides. Also apparently “in Europe and Asia, astigmatism affects between 30 and 60% of adults” (says Wikipedia).
I have it too, and couldn't figure out in the past why text sometimes looks horribly blurry and sometimes doesn't. With this knowledge, I realized that dark background is likely a factor.
So I like dark mode for the OS interface because it gets out of my way then, and just plain looks better. Same in apps where I click on icons and look at images, like in image editors. For reading, dark mode is torture—both with bright white text and low-contrast dark. When sites begin to switch to the dark mode following the browser, I'll have to disable that in the browser settings or deal with bright system and app interface.
However, why I can and do code with dark themes is a mystery. I even tried to bend Emacs to use a light theme for Org-mode and a dark one otherwise (not much luck, though).
I do wish that screens in the ‘light mode’ were closer to paper in terms of eye comfort. I had an e-ink device, and it was a godsend, pure bliss—you'd think that lower contrast would make reading harder but no. Something like HN's ‘black on a bit grayish with a hint of red’ emulates the experience somewhat. Just don't make text gray, please.
In related news, I'm irked that I can't have high monitor brightness for photographic images but lower for swathes of white background under text and in Jony Ive's bright flat windows. Because, you know, photos don't tend to have flood-fill of 100% white. Maybe I'll kludge something up for the browser to amend this.