Thanks for the feedback! We are working on this.
Current Dark Mode support: https://docs.microsoft.com/en-us/officeupdates/release-notes-office-for-macRebecca commented
100% agree with the many comments about changing the reading pane back to white or giving users the ability to save that option once they click the sun/moon icon. I'm currently trying to decide what I value most; being able to view the email reading pane in white or my folders in black - I shouldn't have to make this decision - it was much better before!