White-on-black color scheme
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Originally computer user interface images were formed on CRTs. People likely didn't think much about color schemes in those days. The phosphor was normally a very dark color, and lit up brightly when the elctron beam hit it. The human eye adjusts to make this appear to be green or amber on black, depending on phosphors applied on a monochrome screen.
RGB screens continued along a similar vein, using all the the beams set to on to form white.
With the advent of teletext, research was done into which primary and secondary light colors and combinations worked best for this new medium. (cite...), from a palette of black, red, green, yellow, blue, magenta, cyan and white, it turned out that cyan or yellow on black was typically found to be optimal by test subjects.
The opposite color scheme (black-on-white), was originally introduced in WYSIWYG Word processors, to simulate ink-on-paper.
Some people cite studies to show that a color scheme with white (light) text on a black (dark) background is easier to read on the screen, because the lower brightness causes less eyestrain. The caveat is that most pages on the web are designed for white backgrounds; GIF and PNG images with a transparancy bit instead of alpha channels tend to show up with choppy outlines, and various other graphical elements have other problems.