The reason dark-themed terminal and coding is fine, might be the monospace fonts. They are square-ish, so there is less need to find the edges accurately. They are also eligible on very cheap, trashy screens. As a side effect, code is readable in dark themes even with astigmatism. (No source, just a wild guess)
It doesn't make anything by itself, the difference in brightness between text and background is. I have a severe astigmatism on one eye, HN page with smaller font size is easier to read with it than a green-on-black terminal (same Noto Mono font). Red or blue colors for text on black background are also slightly harder to read due to the perception.
I have good vision: no known issues, regular eye checkups, etc. But I find it nearly impossible to read most displays that use blue LEDs. If the background is blue (eg: timex indiglo watch) then it's fine. But blue leds on some of my home theater equipment drive me insane. Even if it isn't the letters/numbers themselves and it's just a regular circular power led, I find them really hard to look at.
10 years later I developed astigmatism in both eyes. Dark mode is really blurry in some settings.
The blue light thing, as I understand it, is where you first begin to notice astigmatism. The wavelength is shorter and a very slight astigmatism becomes more apparent.