Microsoft Outlook for Windows already supports dark mode with Office Black theme. Even when you enable this theme, reading pane was still displaying text with white background. With the upcoming improved dark theme support, you will also see the reading pane with a dark background when reading emails, and the compose experience with a dark background when writing emails.
You can use the Sun/Moon button on the reading pane or in the ribbon to preview what the message looks like with a light background instead.
This feature is now rolling out to Insiders running Office Version 1908 (Build 11929.20114).