It looks like Microsoft is working hard on improving the dark mode experience on Mail and Calendar app. A couple of months back we reported that Microsoft might be planning to add a toggle in the Mail and Calendar app to switch between Dark and Light mode. The toggle will allow users to easily switch between dark and light mode instead of going into Settings and then changing it.
— Ajith (@4j17h) January 17, 2019
Not only that, Microsoft might be planning to add toggle just for the right panel of the Mail and Calendar app as well. The toggle will allow users to switch just the right panel to dark or light mode. It might come handy for those who like keeping a mix of dark and light mode on their devices. However, there no word on when the feature will make it to Windows Insiders and then to the public.