Thanks for the feedback! We are working on this.
Current Dark Mode support: https://docs.microsoft.com/en-us/officeupdates/release-notes-office-for-macMike D commented
Joining the community - my computer just updated and now I'm having to deal with the same **** of dark background on all my emails. Someone really thought this was a good idea! What the **** people! Give us the option to toggle all emails one way or the other, not click a button for each email. So stupid....