You can change the back ground to dark either dark gray or black. Changing to dark mode also changes for all Office 365 desktop applications. After changing to dark mode when you open an email you can use the sun icon to change the message window to white, you can use it as toggle to switch between dark and white. When sending a new email you can change the message window background to white by clicking on switch background button. If you want to keep the message window always as white even though the back ground is dark you can change the options to keep message window as white always.

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