The Mail app has supported Windows 10's dark mode for some time, but the area where you actually read your email has remained white. That's finally changing, as Insiders on the Fast ring got an update a couple of weeks ago that allowed them to turn that panel dark, and now that update is heading to the Release Preview ring.
Spotted by WindowsUnited, the version number of the new app is 16005.11231.20142.0. Since it's now in Release Preview, that means that a public release is imminent. If you use the app, make sure to keep your eye out.
Microsoft first added the system-wide dark mode to Windows 10 in 2016, with the Anniversary Update (version 1607). Since then, the feature has rolled out to a variety of first-party and third-party apps, and the Mail app even supports it, as stated above. What's new with this update is that it's expanding. One of the last holdouts is OneNote, although that's also changing in a future update.
If you want to download the Windows 10 Mail and Calendar app, you can find it on the Microsoft Store here. You'll only get the new dark mode feature if you're on the Release Preview ring of the Windows Insider Program though.