Step 1
In the upper right hand corner, click on the link “settings”
Step 2
On the right top of the new screen you will see a link titled “Themes”
Click on this link.
Step 3
There are about thirty different themes to choose from. Try different ones out and see which you like best. Some of them will even change during the day. Others are tapped into your location and will change as the weather changes. As you click each choice, you should see the background/theme change automatically.
Step 4
Once you have chosen the Gmail theme or background you would like to use, simply click on your inbox. The change to the background is saved automatically.

nice tips..thnx.