I don't think "dark theme" is always the right answer, but I think with a new medium, a new basic is worth considering.
Reflective media like paper behave a little differently than emissive media like an LCD. A normal sheet of paper under normal lighting conditions won't reach the same brightness as an LCD. An LCD is much bigger than any reasonable book and it emits light uniformly across it's entire area (close to impossible for any piece of paper unless it's affixed on a flat plate). Also most paper is not actually very white.
The goal of design isn't to correct the user, but to correct for the user.
But you can't solve brightness problem with just design of a web page: you can't know for sure that all of your users are in the dark, or that their monitors emit some well known amount of light. Unless, of course, you do natural selection thing by making another category of users suffer and, eventually, leave your page because of your design.
And why is it web designer's job to adapt to myriad of real world devices and conditions? Shouldn't DPIs and automatic brightness correction things in hand-held devices and other OS-side tricks handle this problem already?
Then again, what modern designers are correcting for with, say, slick hair-thin fonts? It only adds extra work: now I need to disable these fonts somehow to make page readable. That's almost like calibrating brightness with a piece of paper in the post above.
However, I've also started to spend more and more time on my eink Android tablet. Video is of course unwatchable, some apps refuse to launch, and scrolling is unpleasant at best. Still, I think the tradeoffs are worth it, especially late at night.