Official Gmail Blog: Choose your own theme colors:
"Since we launched themes for Gmail, many people have asked, not surprisingly, whether they can customize the look of their inboxes without having to use some advanced scripting magic. And while you can't make custom themes that change with the weather like some of the pre-set options, you can now choose your own Gmail colors.
Just click on the Settings link in the upper right hand corner of your screen, select the Themes tab, and find the "Choose your own colors" option at the bottom of that page. You'll be able to select colors for everything from the background of your inbox to link text.
Companies that use Google Apps won't see the options for themes yet, but we're working on getting themes to you too."