This is often repeated nonsense. What is hard on your eyes is changing intensity. Switching between dark code and light email is the problem. The screen should also be about the same brightness as the room, so looking away from the computer doesn't require eye adjustment.
The transition from dark to light is mildly painful, so you have a false sensation that the dark background is better because switching away from it is irritating.
I don't want the full power of the LED backlight to hit me on the eyes with its full force in my editor screen which I look into for 8+ hours a day...
>I can't tell if people coding on dark backgrounds are hipsters, or just don't know any better.
Or perhaps you don't know any better...