I suppose this is already known because it’s really obvious but sometimes the most obvious things get overlooked when you’re “in the zone” (regarding programming an application), so I’m gonna report it anyway:
Does your computer auto-switch from light mode to dark mode at certain times of day, and is Dorico set to follow the system theme? Some apps won’t fully switch from light to dark (or vice versa) until they’ve been quit and reopened, and this has happened to me before since my computer is set to auto-switch and I have Dorico follow the system theme.
Yes, I have it to auto-switch. However, I’ve also switched manually yesterday to rule out this possibility, and it also didn’t change the background; it stayed dark.
It happens regardless of if the switching is manual or automatic. I’m not sure exactly what it is because I know nothing about app design, but some apps and websites don’t fully “refresh” when the theme changes and some parts of the UI don’t switch from light to dark when the rest of the app does. Quitting Dorico and reopening the project will certainly refresh it and make the theme display properly, but it might go away from just closing and reopening the project.