Earlier quoted context omitted.
Most of the data that I've found suggests that dark text on a light background is measurably more legible than the reverse. This effect is increased for people with astigmatism -- which is about half the population, so that's not insignificant. There's a lot of pushback I've seen when studies like that get cited: they're old, they weren't about programmers, etc. And, maybe, but even though we're staring at LCD panels…
I imagine you don't know the answer, but I'm curious as to if these studies were controlled for screen glare. Dark themes/backgrounds have far worse issues with screen glare from badly configured room lighting than light ones do in my experience. ----- I'm also curious why another area of design that is extremely concerned with legibility seems to have reached the opposite conclusion, pretty much worldwide. Road sign…
The road signs are an interesting question. I know there's a lot of work, and occasional controversy, over the typefaces used. If I had to hazard a guess for the color choices, though, it might be the signs have to be very visible in the dark, too -- that's now generally done by using reflective paint for the light part of the sign, but older highway signs, at least in America, used reflective dots in the letters. So that might be a tradition born less out of legibility studies than practicality.