If you are working with Gmail, you can change the theme to a different one anytime. Please see below for details.
Step 1: Login your Gmail account (please refer here to open one if you do not have an account yet);
Step 2: In the Gmail account, click the "Setting" button from the right-top corner;
Step 3: Click the "Themes" command from the drop-down list;
Step 4: Pick up one of the themes from the list and click the "Save" button at the bottom;
Step 5: Alternatively, you can click "My photos" at the left-bottom corner to pick up your own pictures.
Step 6: Go back to your Gmail and you will see a different background.