If you want to undo the Outlook Dark Mode in macOS then follow the given steps:

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<ol>
<li>Open the Outlook client on your macOS device, and select Outlook at the top.</li>
<li>Click on Preferences from the drop-down menu</li>
<li>Under Personal Setting, choose General.</li>
<li>Tick the box for Turn Off Dark Mode, under Preferences.</li>
</ol>