Seems like Microsoft is working on a dark mode option for its Office apps on macOS Mojave.
The product manager for Microsoft Office, Akshay Bakshi, revealed the new option on Twitter this week, hinting that Office testers will get access to this features as soon as next week. The new dark mode for Office will complement Apple's general dark mode in macOS Mojave, but it will likely be limited to Office 365 subscribers only. (What a way to get users to buy into the subscription.)
Once the testing phase is finished with Office 365 testers, it will be broadly available for Office users to subscribe to Microsoft's Office service. While Office 2019 for Mac launched recently, Microsoft is still pushing new features to its Office 365 subscription before any product updates to these standalone licensed apps.
Microsoft’s dark mode update for Office on Mac follows similar dark mode changes for both Outlook.com and Windows 10. Microsoft introduced a File Explorer dark mode with its latest Windows 10 October 2018 Update and has been slowly tweaking its Windows 10 UI over the past year.