Today while checking my mail I noticed the interface of my inbox changed so I refreshed thinking it was an error with my browser. Only to see a link, after the page loaded, telling me I can change my theme by going into the settings area. I was blown away.
This new themes feature was released today Nov.19.2008 and Gmail now has 25 themes (Classic, New Blue, Cold Shower, Steel, Minimalist, Green Sky, Bibblegum, Cherry Blossom, Night Shade, Marina, Dusk, Sunset, Silver Lining, Contrast Black, Shiny, Desk, Tree, Summer Ocean, Phantasea, Zoozimps, Candy, Bus Stop, Ninja, Tea & Terminal) excluding the default and you can expect more to come.
Three of the themes were designed by external designers. These are Phantasea Renewal - Matthew Ferreira, Summer Ocean - Jim Honeycutt, and Cherry Blossom - Vania Sofandi. The rest were done internally.
Go ahead customize your Gmail!