Hi Julie,
I'm new to the Forum (Hi Everyone!) and not too far into GDI, so very little to offer. But, I've been reading re: black backgrounds and text size. Someone already talked about the visual impairment re: color blindness, but another thing you might consider is folks with vision loss from Macular Degeneration (LOTS of us over-50's have this problem).
I don't know about others, but a light colored text on a dark, or even brightly colored (red, bright blue) background is near to impossible to read, unless the text is huge. Also causes severe headaches trying to read them. Plus, many younger people tend to make text size way to small (your text size is great!).
Anyway, just my $0.2
Good luck!
ElisaT