Google-Mail-Snow-Leopard is a project mainly written in Ruby, it's free.
Google Mail, Snow Leopard skin
Originally based on Gmail: Mac OS X Snow Leopard, and now available as a safari extension!
Download the latest version of Gmail Snow Leopard.safariextz
Double click the .safariextz file to install in Safari 5 and follow the prompts
There is no step 3, enjoy!
Mac OS X Snow Leopard idea by M.Mike Kondo (http://userstyles.org/users/20999).
I (Vidal) rebuild this one from scratch for Safari 5.