How to Enable Microsoft Office’s Dark Mode

Step 1: To change your theme on Windows, click the “File” menu at the top left corner of an Office application
like Word, Excel, Outlook, or PowerPoint.



Step 2: Click the “Account” option in the sidebar. On the right, open the “Office Theme” dropdown menu,
and then select your desired theme.



Step 3: To enable dark mode, select “Black” for the darkest possible Office style.

Credit : https://www.howtogeek.com/360470/how-to-enable-dark-mode-in-microsoft-office/