Black text on a plain background elicited reliably faster reading performance than on a medium-textured background. When compared to reading light text on a dark background, people read black text on a white background up to thirty-two percent faster. In general, the greater the contrast between the text and background, the easier the text is to read.
 Research-Based Web Design & Usability Guidelines, 2006 (chapter 11, page 2; http://usability.gov/pdfs/chapter11.pdf )