It is also possible to turn the lights on for both the message preview and composition screens. Similarly, when you compose an email the message body is now black. With version 1907 the message preview pane defaults to black.
UPDATE 10-5-19: For those on the Monthly Channel, you can get dark mode with the 1909 monthly channel update ( released September 30th). However, Office Insiders receive weekly updates, which could be subject to bugs. Note: Anyone can enroll in the Office Insider program. 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. Similarly, when you compose an email the message body is also in white. 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. Some aspects of the black theme were still fully illuminated, such as when you read or composed a message. However, unlike Outlook Web App, or, Microsoft Teams, the black theme was not a true dark mode. The black theme has been around in Outlook for Windows for some time.