alt Hacker NewsTL;DR: because mid-20th-century designers believed soft green reduced eye strain and improved focus.
Basically the same nonsensical belief as in regard the dark mode nowadays.
I don't even believe it's true. Green is just an army colour, that's pretty much it. Army uses army colours. Mystery solved.
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It's the color of plants. A field of grass. Etc.
Maybe it even works better with the color of a clear blue sky above it.
Anyway, it's intuitive and not rocket science.
As the son of a machine tools salesman, I call the article bullshit. Sometimes things just need to be painted and sometimes you just need that WW2 surplus paint to do the job, with the colour not mattering one bit.
With anything, an academic can thread together a theory that neatly joins the dots to sound feasible, but my bet is that 99% of all engineers are stronger at physics than color theory.
I think the reason that I like dark mode is that I have had floaters in my eyes since at least age 14. They stand out against a bright white window background, but I don't notice them at all on a dark window with light text.
Or maybe it's just because that's how IBM PC DOS, BASICA, etc., as well as the VT100, VT220, VT300s that I used did it.
(Also, I think displays should paint with light, and having a white background is painting darkness on a computer screen. It's particularly bad for presentation slides. A light background just screams "PowerPoint presentation".)