I seem to remember that the Xerox Alto defaulted to a white or light grey background as well, and of course the same for early Macs. For me, in the 80s and 90s,

I seem to remember that the Xerox Alto defaulted to a white or light grey background as well, and of course the same for early Macs. For me, in the 80s and 90s, lighter colour backgrounds - even on essentially monochrome displays - were generally a signifier of a higher end, or sometimes experimental, workstation machine or OS, than black backgrounds, which felt much more like corporate droneware. (Of course, that wasn't always the case: the Sinclair ZX81 and ZX Spectrum, both very cheap microcomputers introduced in the early 80s - and I think also maybe the Acorn Electron - defaulted to white backgrounds.)