If that doesn't work, you'll need to update your software

Using apps in dark mode can reduce eye strain and extend your device’s battery life. Microsoft Outlook’s dark mode turns the default white background into black. This tutorial shows you how to use Outlook in dark mode on supported devices.

Open the Outlook app on your computer and follow the steps below.

Please note: Turning on Dark Mode in Outlook will also turn on Dark Mode in other Microsoft Office apps on your PC, such as Excel, Powerpoint, and so on.