Today Microsoft is rolling out an update to its Mail and Calendar app and has finally added a complete dark mode to which the reading pane is now dark (previously the reading pane or email body was white). Microsoft has also added a toggle button to switch between light and dark which sits between the Forward and Archive buttons.
The new update is available for everyone that is using Windows 10 1809 build. You can download the Mail and Calendar app from the Microsoft Store.
Enjoy!