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
The only thing you have to do is to make this toggle "Switch Background" button global, now every time you open an email settings turn to a default of Dark Background if you are using Dark theme.
731 votesAnonymous commented
+1 "Switch Background" needs to be an universal toggle for all the emails. It is switching back to Dark Mode as you move to a new email.