Bizarre (but then it is PBase), I tried the "Drop & Drag" editor which I normally use, and also the "Original" editor. In neither case do I see a color choice (maybe I need detailed directions to the command/button sequences to get there), so I tried switching to "No Style Sheet" thinking that might pop up different options, but still see no color choices. When I do that to one of my galleries, I get a black background. It appears there is a default .CSS file that loads, and then the customized user .CSS file, if there is one (which I normally use). If I look in "View source" and grab the default CSS file, I see some color specs, but even then what I see doesn't quite make sense -- and I see some of the same color settings from the default .CSS via your gallery or mine -- but the background colors are opposite!
I am not truly a web programmer, but I have at least a little working knowledge of it. Mayhaps I am misunderstanding the CSS code, something I know the least about. Or maybe Firefox 50.1.0 works differently than your browser?
(Methinks the smart machinery is beginning to outsmart itself!)
Good Luck -- maybe someone else here will have an idea. (Richo?!!!)
DaveT