I think the switch to black-on-white when GUIs first appeared was largely because the prototypical application was a word processor, where the selling point was

I think the switch to black-on-white when GUIs first appeared was largely because the prototypical application was a word processor, where the selling point was that what you saw on the screen looked like what you were going to print on paper. Other applications, like spreadsheets, ended up with a lot of the the same printer-oriented design. It showed up in things like font sizes in points (which are nominally length units, but were usually based on some fixed notional dpi rather than matching the size on the physical screen).