It's not because I think it looks good. I do it on principle.
Imagine that the road sign above was inverted, so that all the lit LEDs switched off, and all the off LEDs were switched on. That would be the equivalent of a computer screen with black text on a white background. As the sign actually appears in the photo, it's equivalent to a screen with light text on a dark background, because the background pixels are "switched off", and the part you want to read is switched on. Paper and computer screens work differently because paper reflects light, while computer screens emit light.
The main reason we use white backgrounds is because it aesthetically resembles printed paper. And many claim it's easier to read that way. I personally find the opposite to be true, and I think perhaps people are just conditioned to think that way. I don't really know for sure.
So it's dark here because that just makes more sense to me. Even though many consider it to be bad design.