As this is a suggestion which comes up quite regularly, I felt it valuable to document some of the research I have collected about the readability of light text on dark backgrounds.
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 Columbia
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.