MacOS Mojave release includes a system-wide Dark theme at Apple | System Preferences | General | Appearance. Compatible programs will switch to darker menus etc. We send it out every night around 9 PM Eastern Time (USA). Microsoft recently released Dark mode option for the Microsoft Office suite, along with dark grey and colorful themes. Light: Use the light appearance.Dark: Use the dark appearance.Auto (available in macOS Catalina): Automatically use the light appearance during the day, and the dark appearance at night.See More.
Originally Office 365 for Mac did not change with Dark Mode but now it does.