You can ‘go dark’ with Office 365 for Mac combined with the MacOS Mojave Dark Mode.
MacOS Mojave release includes a system-wide Dark theme at Apple | System Preferences | General | Appearance. Compatible programs will switch to darker menus etc.
Originally Office 365 for Mac did not change with Dark Mode but now it does.
Light mode in MacOS gives the more traditional Office look.
Office 2019 misses out
Even though Office 2019 for Mac has just been released, it misses out on Dark Mode.