Last year at Ignite Microsoft announced Dark mode for Outlook On the Web , while this seem to excite a lot of people I never really caught the buzz. However after taking the plunge after being notification bugged by Outlook this week I've found it to be a nice addition especially if your eyes aren't 100%. When you enable Dark mode using the slider in Outlook on the Web This changes/creates a setting called " isDarkModeTheme " in the OWA.UserOptions User Configuration Object which is held in the FAI collection (Folder Associated Items) in the Non_IPM_Root of the Mailbox. If you want to enable this setting for a user (or users) programmatically or just want to take stock of who is using this then you can use EWS to Read and Set the value in the OWA.UserOptions User Configuration Object in a Mailbox. (if you want to do this in the Microsoft Graph you will need to cry into your beer at the moment because the Microsoft Graph still doesn't support either user
Pushing the Envelope in Messaging and Office 365 Development