Thanks for the feedback! We are working on this.
Current Dark Mode support: https://docs.microsoft.com/en-us/officeupdates/release-notes-office-for-macRuss commented
I actually like the having the Preview Pane dark in Dark Mode. Please add the toggle switch for those who don't but please don't just make it white. I hate that when I create a new email the background is still white.
I'd also prefer a toggle to make the background Dark with White fonts for new email as well but have it sent with a White background with Black Fonts.