Mine's done for the opposite reason.
Black-on-white (or vice versa), or close equivalents (black on light grey, as an example) are not naturally easy to read. You read using the very centre of your retinas (the fovea), where there's the highest density of cone cells (about 70/micrometre) and lowest density of rod cells (just about none). Cone cells distinguish colours from one another, rod cells distinguish light and dark (and are much more sensitive to it).
That's not to say colour is automatically easier to read than black - strong, primary colours, particularly bolded and red, are quite the eyestrainer. The fovea does contain lots of yellow pigment though, making blues that little bit more natural than other colours (it's part of a natural phenomenon of lenses called "chromatic aberration" - different colours are focussed on different parts of the retina).
I tend to, on long posts with significant information, debold my text except for headings. That's just for stylistic reasons - it's hard to emphasise headings properly if everything else is emphasised. I don't tend to change the font as that's really only a personal choice of style. I like Calibri and Gill Sans but they don't work well at 12pt. Verdana too - but that's close enough to the forum's native font.
As for how it started... I first got "into" the GT community on GT3Times and that board had a very dark grey background and burgundy text that I found quite difficult to read at times, so I went for a bold cyan (again with the blues!). When I came here I tried to find a nice, soft, easily-perceived colour to stick with the same theme.