Office 365 (Outlook on the Web) - Change the Theme
A theme, which changes the background color, highlight colors, and images, is a quick and easy way to give a different look to Office 365. This article references the NEW Outlook on the Web.
Select Settings 
, and in the Theme section, select a different theme. Click View all to see more choices.
, and in the Theme section, select a different theme. Click View all to see more choices.