Themes allow you to control the overall look of Gmail without changing its functionality. Gmail provides several default themes, and you can even create your own.
To change your theme, click the wheel drop-down icon and Settings.
Click the Themes tab, then click the Set theme link.
The default theme is set to Light.
You can choose any theme here or click to choose a photo as your theme.
Let’s pick the Blue theme.
That’s it! The theme has been changed.
When finished, click Save.
This is the end of the tutorial. You now know how to change your theme in Gmail.