I was surprised when I logged into my email today, since I usually check using IMAP. The site colors suddenly changed to light blue, with a notification message stating that new themes are now available on Gmail. I really didn’t like the default set of theme on my mail, so I dropped by the themes section to change.
I was shocked to see that Gmail Themes doesn’t only feature themes with different shades of colors, but themes that look fun and modern like the Ninja theme (that’s what I’m using now). Their themes also have some cool tricks, like changing the way they look based on the time, so they ask for the user’s Location to synchronize the time. Now, I’m excited to see how my theme would look later.
Themes are not available with Google Apps, though I’m hoping that Google will include themes to the Google Apps, for my ChanLu and PixelMai mail to look more fun.

Not only themes, they also have useful gadgets in the Labs tab.
You can also skin you gmail using this add-on in firefox:
https://addons.mozilla.org/en-US/firefox/addon/6076