After some testing with Office Insiders, the December 2018 update for Office 365 for Mac is now bringing dark theme support to all Mac users on macOS Mojave. If you have already enabled the system wide dark theme on the latest version of macOS, Word, Excel, Powerpoint and Outlook will automatically switch to dark mode once you install the latest Office build.
In addition to dark mode support, the December 2018 update also introduces new proofreading tools in PowerPoint, as well as the ability to insert a photo from your iPhone into your slides thanks to the Apple Continuity Camera feature. Outlook for Mac is also gained several new features including calendar sharing, the ability to use Microsoft Teams to connect to your meetings and more.
There are no new features for OneNote for Mac this month, and the free note-taking app isn't getting dark theme support, for now at least. If you want to get early access to new features, don't forget that you can join the Office Insider program right from Microsoft's AutoUpdate app.