Before You Begin
Opening the Inbox is like walking into your office: you're going to be there for a while. You can customize the Theme in Office 365 if you wish.

Try It: Change the Theme
Go to the Office 365 portal:
https://portal.microsoftonline.com.
Please log in.
Go to Home>Settings.
Click on Change theme.

What Do You See? The Theme gallery will open. Select a Theme, then click OK.

When you click OK the Theme will be applied to the Inbox.

Cheers!

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