The black theme has been around in Outlook for Windows for some time. However, unlike Outlook Web App, or, Microsoft Teams, the black theme was not a true dark mode. Some aspects of the black theme were still fully illuminated, such as when you read or composed a message.
The screenshot below is an example of how the black theme changes certain aspects such as the folder view and toolbars to black but leaves the email preview pane in white.
Similarly, when you compose an email the message body is also in white.
The new Black Theme for Outlook
Starting with version 1907 for Office Insiders, the Outlook team has extended the black theme so it now incorporates the message body and the composition panes.
Note: Anyone can enroll in the Office Insider program. However, Office Insiders receive weekly updates, which could be subject to bugs.
UPDATE 10-5-19: For those on the Monthly Channel, you can get dark mode with the 1909 monthly channel update (released September 30th).
With version 1907 the message preview pane defaults to black.
Similarly, when you compose an email the message body is now black.[Read more…] about Improved Dark Mode in Outlook for Windows