Gmail theme editor lets you modify background colors, text colors, links colors, and background image. It is so simple. Here is how you can do that.
Login to your Gmail account, then go to the Settings - Themes tab and choose “Create your own theme.”
From the custom color panel, select background images for the main area and the footer. You can select images from your Picasa album (if you have no Picasa account, create one, and then upload images).
You can also change color of texts and links from the custom color panel. After making changes, save settings.