If you're using the beta or dev channel for Google Chrome, you are now able to install themes from Google's theme directory. To install a theme, you simply have to click the button -- it couldn't get any easier.
There are 29 themes currently available in Google's directory, and they are all actually pretty good. I assume Google will allow third party developers or designers to create their own themes for Chrome eventually.
My favorite theme right now is "Glow" -- what's yours?