When the Windows 10 update arrived 3 years back it brought the ability to enable dark mode within the window of the app. The modern UWP apps in Windows 10 can use dark windows background instead of white.
In Windows 10, there are two ways to enable dark mode in apps. First, you can open the Settings app and enable dark theme under personalization setting. You can also enable a dark theme on certain apps from the setting page of the apps as well.
Windows 10 Mail app allows users to switch between dark and white theme directly from the app’s settings page. Microsoft now appears to be testing a new update for Windows 10 Mail and Calendar app that would add a toggle that will help you quickly switch between dark and white mode.
Microsoft is adding this toggle to the header of the app and you can use it to toggle between dark & light theme in the Mail app.
Mail app to get slightly lighter dark mode
Windows 10 Mail app is also getting a new dark mode that would look slightly less dark. In the latest Insider build, the app appears to use a dark grey background colour instead of pitch black.
Microsoft is testing the updated dark mode with select Windows Insiders and this update will completely abandon the pitch black background colour in favour of lighter dark mode.