There should be a dark theme and the option to follow the system theme
The App AutoDarkMode allows automatic dark/light windows system theme switching: GitHub - AutoDarkMode/Windows-Auto-Night-Mode: Automatically switches between the dark and light theme of Windows 10 and Windows 11
OneCalendar should provide a dark theme and the option to follow system settings so that it also switches automatically.
This can be a premium feature (autoDarkMode users will pay gladly ;)