I’m running Rhino 6 WIP on Windows 10 (post creators update) and have not been able to change the Windows menubar color that carries across from app to app. Which was possible in Windows 7 via the appearance settings editor and/or the registry editor.
Perhaps you could add/override this color within Rhino via File > Properties > Appearence > Colors settings?
Cheers and best,
(Updated for clarity)