This happens to me while my Mac is in Dark Mode. (I think the issue is that Catalina is smart enough to automatically change the cursor color to white while Dark Mode is active, but it doesn’t/can’t change the background color of the authoring pane.)
The developers have been made aware of the issue so hopefully a future release of Inform 7 will include an incredibly slick dedicated Dark Mode interface.
In the meantime, what I do is: Exit Dark Mode when I’m using Inform 7. I only have Dark Mode active at night, but from your post it sounds like maybe you have it active all the time, so that’s somewhat more inconvenient.
Using this tutorial I set up a keyboard shortcut for toggling Dark Mode, which should make things a little more convenient. (It always toggles back on its own when I leave my computer for a few minutes, but you might not experience that if you don’t have the time-based activation set up.)