I’m experiencing this now too. This is a new issue for this particular website that has a dark background set via GP customiser. I have updated to the latest GP theme and doesn’t solve the issue. I also tried changing the background color of the editor via a editor stylesheet but it doesn’t seem to register…
can you raise a new topic where you can share your site URL ( to a page where the problem exists ) in the Private Information field provided – then we can take a look.