It appears as though Office 365 subscribers will get first dibs on dark mode — Microsoft tends to release Office features on that product before they make their way to the standalone Office apps. The update mirrors Mojave's own dark mode, while Microsoft added a dark mode to File Explorer in its Windows 10 October 2018 Update.
📣 Office Visual Refresh and Dark Mode support rolling out now to Insiders Fast with build 181029!
— Akshay Bakshi 🎉 (@AgentAkki) October 30, 2018
Update 10/30/18 5:20PM ET: Dark mode is now live for Insider Fast users. This post has been updated to reflect that.