Addition of color in the background and the font, of individual characters. In addition you can change the use of a custom font. The defaults are just a demo, they weren't thought through.
Potential uses with people of low eye site (read with out glasses), near blind (increase screen letter density), dyslexics, synaesthetics, children learning to read, as mnemonics.
Second screenshot is for comparison, if the second screenshot was with normal black and white fonts, it would be totally impossible to decipher anything.
related greasemonkey script (+images)
Software implementation proposals (SVG fonts)