To change your theme in Gmail, first log into Gmail and go to Settings in the upper right-hand corner.
Next, click on the Themes tab.
Choose a theme by clicking on one of the theme thumbnails. If you don't like any of the themes, you can also choose your own color scheme.
Make sure you set your location if you choose a theme that changes with the time of day, such as Tea House.
That's it. You don't need to press save anywhere to keep your changes, and you can go back and change your theme as often as you'd like.