In the bad old days before ubiquitous color bitmapped displays, people used monochrome monitors that often used green on black or amber on black. More likely than not, jwz's color scheme is a homage to that.

I really don't get why there is so much complaining about this every time someone posts a link to jwz's blog. Personally, I don't find it difficult to read at all. If anything, I prefer light text on dark background.