Originally, computer user interface images were formed on CRTs. The phosphor was normally a very dark color, and lit up brightly when the electron 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 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. 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.
There seems to be a dispute among vision and perception researchers about whether it is actually easier or healthier to read text on dark or light background; there is a similar dispute between users, when using each other's computer terminals.
Some argue that a color scheme with light text on a 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 transparency bit instead of alpha channels tend to show up with choppy outlines, as well as causing problems with other graphical elements.
It is not necessary that a web design work well with only one color scheme. There are many mechanisms of web architecture that allow designs to work well with any color scheme a user might prefer. This technical flexibility is a product of the web architect's concern for accessibility and user preference empowerment, though designers rarely utilize this technical flexibility.
Unlike paper which reflects ambient light, both CRT and LCD displays emit light of sufficient brightness to overcome ambient light. As ambient light varies, the relative brightness of the display can vary widely.
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