There are two ways to enable dark mode:

Option 1: System Settings

You can enable dark mode from your system settings (Windows, Mac, iOS, Android, etc.), and OneNote will automatically adjust. If OneNote was already open when you enabled dark mode, make sure to restart the program or app for the changes to take effect.

Option 2: OneNote

Click on "View" in the top menu and select "Switch Background". With this method, only the page backgrounds will be in dark mode while the rest of the app remains in light mode.