According to Official Gmail Blog, they are rolling out almost 30 themes for gmail. These a shiny theme with chrome styling, another one that turns your inbox into a retro notepad, nature themes that change scenery over time, weather driven themes that can rain on your mailbox, and fun characters to keep you in good company.
This makes Gmail's default look crisper and cleaner -- you might notice a few colors and pixels shifted around here and there. You can see some of the themes in below snap.
To customize your inbox, go to the Themes tab under Settings. Currently themes are not available to everyone and will be rolled out to everyone over the next couple of days.