Well, this is a mess of bad information.
This being the top answer with 10K upvotes is frustrating.
First, you can strain your eyes looking at white pages in a bright environment. White paper is around 80-90% reflective and the sun is very bright. Eye strain existed before screens did. Eye strain does not care if light is emitted or reflected, just how bright it is.
This all has to do with how our eyes adjust to different lighting environments. When things are bright, our pupils get tiny and restrict light. When things are dark, our pupils get big and suck in more light.
First, why do we need dark mode for screens?
In the case of reading text, we need good contrast in order for it to be quickly legible. Contrast is a ratio from the brightest thing to the darkest thing. The bigger the gap between the text and the page, the less work your visual system has to do to resolve the letters. So black on white and white on black provide a good reading experience for most people.
If we are reading a screen in a dim environment, our eyes are mostly adjusted to dim lighting. But if a screen is emitting a bunch of white light at full blast into your eyeballs, that can feel intense or straining.
You can turn on extra lights so the room brightness is roughly the same as the screen brightness.
You can enable dark mode so there is less total light overwhelming your eyes while retaining a good legible contrast ratio.
Or you can dim your screen to match the dim environment.
The last option can be problematic depending on the type of screen you have. Even good LCD screens will have a reduced contrast ratio as you turn down the brightness. White will turn into dull gray and the contrast you need to easily read text will be diminished.
If you have an OLED screen, you may be able to turn down the brightness quite a bit more before you lose that high legibility. OLEDs sidestep this because the dark pixels emit no light at all, so the contrast ratio is more robust at different brightnesses.
Physical books have the opposite problem.
The brighter the environment, the brighter the pages. If you are in bright sun, a "dark mode" book will reflect less light and be less intense while maintaining a good contrast ratio for text.
However, if you take it in the shade or a dimmer environment, there may not be enough light to properly illuminate the white letters.
A book with white pages is going to have more utility. You can read it in both dark and bright environments. And if it is too bright you have options like sunglasses or finding some shade. For a dark mode book, you may have to add more light to get good legibility.
There are some folks with certain eye conditions that could really benefit from a dark mode book. For them, I'd probably recommend an e-ink display that can invert black and white.
I don't think making a dark mode version of physical books is practical. But the concept isn't "dumb."
