Windows 10 dark mode can best be described as a journey, not a race, and that's evident with its default Mail app. For years, the app was able to go dark except for the actual page where you read the email, which remained white. That's now changing as spotted by WindowsUnited.
Starting with version 16005.11231.20142.0 for those in the Windows Insider Release Preview Ring the Mail app can now go completely dark (a restart or toggling the dark mode option may be required).
The new dark mode, along with a bevy of other UI changes, were first spotted on January 24, but only for those in the Windows Insider Fast Ring ("19H1"). Now, those on the October Update (1809) can get the new version as well.
While not a massive overhaul it's these little things that go a long way to making the native Mail app on Windows 10 rather fun to use.
There's no word on when this update will go to those on non-Windows Insider builds (aka Production), but with this final stage of testing, it is likely just a few weeks.