From the Microsoft post:
You asked, we listened! Dark Mode is now rolling out in the Office app for Android phones. It’s been a highly requested feature by many of our customers, and we’re proud to begin releasing it today. Many people prefer using Dark Mode as they find it provides a more comfortable visual experience for reading and working on mobile devices.
The Office app helps you get work done on a mobile device by combining Word, Excel, and PowerPoint into a single app and adding many rich capabilities for creating and working with PDFs, images, scanning, capturing notes, and much more. Now you can do all this using a theme of your choice — based on the time of day, your surroundings, or your device settings. Dark Mode ensures that you can adjust the Office app to low-light environments, while providing you with better viewing options and keeping battery consumption low.
The newest version of the Office app for Android will automatically adjust to Dark Mode if your device is already set to that preference. You can also turn Dark Mode on or off by tapping your profile icon on the Home tab in the Office app, selecting Settings, and then selecting Light, Dark, or System Default from the Theme options.
Download the Microsoft Office Mobile App today and always keep it up-to-date to ensure you have the latest features and improvements. The feature will be rolling out to users worldwide over the next few weeks, so if you do not see it in your app yet, you can expect to receive an update in the near future that will have it.
Please share your feedback with us about Dark Mode right from the app by tapping your profile icon on the Home tab, and then select, “Send Feedback.”
For more information about Dark Mode investments across Microsoft apps, see also 'Designing Dark Mode'.
A few screens of this in action on my Android-powered Samsung Galaxy S20 FE:
The system's not perfect (yet), with the loading screen for a document flashing white briefly. Argh, my eyes - at least potentially! I'm sure this will get quoshed to keep white elements away in a further update.
Of course, Windows phone users have had dark everything for a decade, then iPhone owners were gifted dark themed Office in 2020, and now Android users can join in preserving their retinas when happening to open a Word file at 1am!