It’s the default Dark theme. I switched to Light theme and it’s the same.
EDIT: I’ve switched some themes around - now on “Dracula” – which doesn’t have this problem. I think flipping the themes around fixed it for me. As to how the issue for me started, I started first to create notes on the Mac client and had a theme - Dracula - installed. When I loaded my synced notes on Android for the first time, it was on Light mode + some theme I didn’t recall. So switching the theme fixed it for me.