Hey @Jackie_ - Two possibilities come to mind - one (something I encountered on the Mac) was that I had set something in the styles group of preferences to white or something. The other option (that I can think of - I'm sure there are probably more) is that there is something about the windows theme that you are using that might be causing a conflict.
To address the first possibility you can go into styles and there is a button that says "original" that should reset everything.

To address the second, you might need to contact tech support (support@jmp.com) to get some help unraveling what might be going on.
(Edit: you might also have a look at this thread in the community - it looks like a similar problem to yours but in reverse: https://community.jmp.com/t5/Discussions/JMP19-Stuck-in-Dark-Mode/td-p/905568)
Hope that helps!
Best,
M