Thanks for the feedback! We are working on this.
Current Dark Mode support: https://docs.microsoft.com/en-us/officeupdates/release-notes-office-for-macAnonymous commented
I agree with a lot of below comments - I love dark mode for the message bar on the right but prefer light mode for the message pane itself. Would love to see it as a preference instead of a toggle on each individual email (that switches back when I click off that email, even!).