That depends on the background light in the room. In a dark room, light on black is the most readable.

Light on black is wretched for people with astigmatism.

Astigmatism in both eyes and I still use it although my glasses help somewhat. I find that it's usually not all that bad at night when it comes to the halos around text or doesn't really bother me enough vs adding more light by going with a lighter grey theme.

I have pretty bad astigmatism and love white on black in a dark room on an AMOLED screen with the brightness turned way down. It's the only screen I can look at for over 10 hours without getting a head ache.

I happen to have a pretty pronounced astigmatism. I still prefer light (note, not white in the case of my site) on black.