You can change your Outlook theme in your account settings while logged in through a browser or in the Mac and Windows apps.

To turn on dark mode in Outlook, click the File tab, choose Office Account, and set the Office Theme to black.

Here is a complete guide to on how to add or change the profile picture of your Outlook account on Windows 11/10. You can follow this post if your Profile picture is not showing up in Outlook. How to ...