In Outlook for Microsoft 365, the black color theme includes dark mode, which provides a black background (instead of white) for the message window. You can switch to a white background for the message window if you prefer.
Switch to black theme
Dark mode is enabled by default when you use the black theme. To switch to the black theme in Outlook:
Go to toolbar and select File, then select Office Account.
Under Office Theme, select Black.
Select the back arrow at the top to return.
Note: The theme you choose will be applied to all Microsoft 365 desktop applications.
Toggle between dark mode on and dark mode off
When reading a message, you can turn dark mode on or off by using the icon at the top of the message window.
Select the sun icon to change the message window background to white.
Select the moon icon to change the message window background to black.
When composing a message, you can turn dark mode on or off from the Message toolbar by selecting the dark mode icon to change the message window to white or black.
Disable dark mode
If you prefer, you can disable dark mode so that the message window is always white when using the black theme. The icons to switch dark mode on or off are disabled when you disable dark mode.
In Outlook, go to File > Options.
On the General page, look for Personalize your copy of Microsoft Office.
Set the Office Theme to Black and select the check box next to Never change the message background color.