https://wccftech.com/microsoft-office-mac-dark-mode-mojave/

With the latest feature update rolled out to macOS in September, Apple introduced a system-wide Dark Mode for Mac users. Microsoft is now working on a dark theme for its Office apps running on macOS Mojave. Akshay Bakshi, Microsoft Office product manager, revealed the new feature on Twitter earlier this week. Office Fast ring testers are the first to get access to this Dark Mode.

Look closely at the Pictures icon. Office running on macOS Mojave in Dark Mode. #OfficeInsiders #office365 pic.twitter.com/xDSTFvNr5q