The Office team plans to announce a big landing tomorrow. Based on the posters released by some Office team members in advance, it seems that Microsoft is providing Dark theme support for Outlook and other Office applications. Outlook design director @milesfitzgerald wrote in the tweet:
There’s something big landing for @Outlook and the rest of the Office apps. We’ve crafted every single part of the app on iOS/Android. I can’t wait to share more tomorrow. ✌️ pic.twitter.com/COwRAOO5SV
— Miles (@milesfitzgerald) August 27, 2019
Then another product manager, Michael Palermiti, who is responsible for iOS Outlook, forwarded the tweet @milesfitzgerald and left a message (now deleted): “One switch switches everything.” implies that outlook and other Office applications will greet Dark theme support.
It seems like Microsoft was working on this change for a while. It was listed on Microsoft’s 365 roadmap page as a work in progress.
Outlook for Windows: Support for Black Office Theme with reading pane toggle
Office Black theme will turn all Outlook screens into dark mode and adds a sun/moon toggle between black and white based on your preference for reading message content.