After 4 years of the plain old blue and white, the natively supported themes section of Gmail is a huge welcome. It's about time guys and great job with the themes.
It also looks like they tried to please everyone. For the hackers among us, we have the Terminal Theme and for the kids, there is the Candy Theme. All the themes are very well thought out, creative and again, a sight for sore eyes. I was starting to think that they had given up on aesthetics and were aiming purely for functionality...not that it's a bad thing, but you shouldn't overlook the visual aspect of a great application either.
I'm loving these themes, and I'm sure it's a matter of time before we are able to create our own themes, with custom pictures and colours.
Also, with the introduction of Video and Audio chat, Gmail is leaving the other mail 'websites' in the dust. When you bundle all these into a package that is also tightly knit with Google Calendar, Google Docs and Google Reader, it's a great recipe for success.
Keep up the good work Google...