You can change the look and feel of Office 365 by choosing a theme that fits your personal style or mood. Themes control the color and appearance of the nav bar at the top of the screen, and the color of highlighted text.
Change the Office 365 theme
The Office 365 theme sets the color scheme and backdrop for the Office 365 navigation bar at the top of the screen across Office 365—including Outlook, your Home page, Yammer, OneDrive, and Sites. It also changes the color of some of the tiles to match.
On the Office 365 navigation bar at the top of the screen, click the Settings icon .
Click a theme in the gallery to see it previewed on your screen. Scroll down and click Save to set it as your theme.