1-Open Outlook 365.
2-Click on the "File" tab.
3-Click on "Options".
4-In the "General" tab, click on the "Personalize your copy of Microsoft Office" section.
5-Under "Office theme", select "Black".
6-Uncheck the box next to "Never change the message background color".
7-Click on "OK" to save your changes.
This will change the color scheme of Outlook to dark mode, but the background color of messages will remain the same.
Here are some additional details about how to turn on dark mode in Outlook 365:
On Windows, you can also turn on dark mode by clicking on the "Start" button and searching for "dark mode". Then, click on the "Turn on dark mode" setting.