The science of readability is by no means new, and some of the best research comes from advertising works in the early 80s. This information is still relevant today.
First up is this quote from a paper titled “Improving the legibility of visual display units through contrast reversal”. In present time we think of contrast reversal meaning black-on-white, but remember this paper is from 1980 when VDUs (monitors) where green-on-black. This paper formed part of the research that drove the push for this to change to the screen formats we use today.
However, most studies have shown that dark characters on a light
background are superior to light characters on a dark background (when
the refresh rate is fairly high). For example, Bauer and Cavonius
(1980) found that participants were 26% more accurate in reading text
when they read it with dark characters on a light background.
Reference: Bauer, D., & Cavonius, C., R. (1980). Improving the
legibility of visual display units through contrast reversal. In E.
Grandjean, E. Vigliani (Eds.), Ergonomic Aspects of Visual Display
Terminals (pp. 137-142). London: Taylor & Francis
Ok, 26% improvement – but why?
People with astigmatism (aproximately 50% of the population) find it
harder to read white text on black than black text on white. Part of
this has to do with light levels: with a bright display (white
background) the iris closes a bit more, decreasing the effect of the
"deformed" lens; with a dark display (black background) the iris opens
to receive more light and the deformation of the lens creates a much
fuzzier focus at the eye.
Jason Harrison – Post Doctoral Fellow, Imager Lab Manager – Sensory
Perception and Interaction Research Group, University of British
The "fuzzing” effect that Jason refers to is known as halation.
It might feel strange pushing your primary design goals based on the vision impaired, but when 50% of the population of have this “impairment” it’s actually closer to being the norm than an impairment.
The web is rife with research on the topic, but I think these two quotes provide a succinct justification for why light text on a dark background is a bad idea.
More on the subject - http://blog.tatham.oddie.com.au/2008/10/13/why-light-text-on-dark-background-is-a-bad-idea/