I've sofar only seen this with the confirmation dialog when aborting "Deleting tags".
I've set MP3Tag to dark mode, but the dialog comes up in light mode.
I dont know if other dialog prompts do the same, but I've no idea which actions generate one. I just don't use MP3Tag often enough. (I actually do most of my tag editing in Foobar2000)
AFAIK those message boxes are default system message boxes which follow the system color scheme.
I would honestly not know for sure, to be honest, I never payed that much attention to them. You could be right though. I'll see if I can force one outside of MP3Tag.
I just tried to restore a file from the recycle bin and this is what it looks like:
