That looks the inherent Qt dark mode. I've been having trouble getting the SIGIL_USES_DARK_MODE variable to behave lately. I'm actually trying to debug it right now!
I'll see what I can do. But it's getting harder and harder to force a light/dark theme that goes against the system/app setting with Qt on Windows.
In the meanwhile, you should be able to choose a custom color scheme on Windows: Apps dark/System light (or vice versa) that works well for me (with these latest Sigil betas). I realize that's a system-wide setting and not the Sigil-only override you're looking for, but we'll see what I can do.