If I encounter a page that's dark mode (GUI), after about a minute or so, I start seeing spots before my eyes that stay with me for quite a while.

To wit... I use these two firefox extensions to convert pages from Dark Mode to Light mode. If one fails, the other usually finds success (I think they work in reverse too):

https://addons.mozilla.org/en-US/firefox/addon/site-color-ch...

https://addons.mozilla.org/en-US/firefox/addon/no-color/

side note: I cannot explain why the terminal (FG/White; BG/Black) feels incredibly comfortable to my eyes. Maybe it's the larger font I use (80 columns = ~2/3rds of my 16" screen width), the right amount of contrast, the fixed spacing...?

And the content on the screen felt like real-world tangible object.

Having said that, I feel that dark mode (full dark) is a product of "new is exciting", "game theme", "dark means serious, mystery, suspense", "no glaring", "linux/console style, so geek", "we are coders, so different from normal people" etc

The reality is, full dark theme requires more brightness and more power consumption. It feels like searching for things in a dark room without light (mystery, suspense factor). Light theme can feel more relaxed, with lower brightness settings.

Color schemes were always supposed to be infinitely configurable by the user, not something a third party can control and dicatate. There used to be no concept of dark or light "mode", every person configured their UI colors as they wished. Want green on pink mode? No problem, just configure it.

(But to answer the question, definitely "light mode" for me. A predominantly dark screen brings on a headache quickly.)

So I've set up my desk at a 45° angle bridging a corner of my office where I have windows along one of the walls (with me looking at the corner, of course, not into the room).

So, I'm as close to the windows as possible and have a wall of windows with natural light hitting my eyes from a 45° angle. This is important on overcast days which, where I live, is most of them because otherwise, I would get barely any natural light at all.

With dark mode, this setup runs into trouble on sunny days because sunny spots in the room tend to reflect off the screen, producing glare. This leads me to start drawing blinds and almost always spirals into me sitting in a dark room on the sunniest of days when I most want to enjoy the sun.

With light mode, I can get away with regulating the light through a movable paravent that moves with the sun and takes care of the worst of sunny spots reflecting off my screen and maybe drawing the blinds only partially.

With such an extensive time study, highly unlikely to be usurped any time soon, and the result being quite the opposite of what I set out to prove, we can all safely put this debate to rest.

A contributing factor is my keratoconus has had some kind of remission. Which is a good thing. At one point I had so many focal points in both eyes, without corrective lenses and before surgery, more than a dozen overlapping versions of text produced unreadable spaghetti. Unless it was a very tiny font from a distance, and then the glyphs only partially occluded each other, and I could I decode them.

Looking at one of those small bright electronics power LED dots across a dark room, I could see all my focal points in each eye, and focal webs meandering between them.

So I feel quite privileged to be able to use dark mode with unaided eyes now.

Edit: I underwent LASIK eye surgery, and I don't recall experiencing headaches beforehand. Or maybe, just getting old?

If your monitor is so bright that a white rectangle hurts your eyes, you need to turn it down.

On my current monitor I go from 100% brightness during the day to around 20% at night. I change it roughly twice a day.

I don't understand how, even with dark mode, someone could run a modern panel full tilt at 100% through the evening. My brain can sense the intense heat of that backlight behind every pixel. I don't even want the possibility of a momentary dark mode glitch to be a concern. It took me a long time to appreciate the harm of not controlling this stuff.

Staring into a light source that contrasts enough with the ambient light to contract my pupils is uncomfortable. I don't want to do that even if it makes me read faster.

Historically, printing black text on white paper was presumably technically easier or cheaper, but I doubt this makes much difference for modern printing of the last fifty years or so.

Edit:

Thinking about it, it's clear that dark mode is most pleasant in dark environments, because it reduces the contrast between the screen and the environment. However, books, being reflective rather than emissive, don't work in dark environments at all. Which defeats the main use case of dark mode.

Moreover, since books are reflective, there is no risk that they get brighter than the environment, while this risk exists with light mode on screens with a brightness level set too high.

Dark mode at high brightness and light mode at any brightness on LED screens both give me migraines.

That said, light mode on non-emissive (e-ink, actual paper) is find.

night → dark mode with high brightness ⇒ less eye strain

When people talk about light mode blinding them, please do yourself a favour and do not live like a goblin. Work in a well lit room and calibrate your monitor, your eyes should not be hurting looking at bright colors.

Seriously, the best UIs let users adjust things to their preferences instead of forcing one or two-polar-opposite choices.