Though Gmail has built-in feature to theme your Gmail, you will me amazed to see this metallic theme achieved through a tiny Firefox extension called, Google Redesigned. It not only redesigns the Gmail look, but also the login form, Google Calendar, and Google Reader. If you have a login ID at Mozilla, you can download the extension from Mozilla Add-Ons Repository here: https://addons.mozilla.org/en-US/firefox/addon/8434/. Otherwise you can go to the author’s site and directly download it.
Description from Mozilla:
“Google Redesigned is a Mozilla Firefox extension designed by Globex Designs that aims to fully redesign the look and feel of popular Google services. This is achieved with Cascading StyleSheet (CSS) files which are loaded on the client’s browser. The extension simplifies the use of these styles by providing auto-updates, easy management and notifications of changes. You can also download and use the styles individually by going to their respective pages.”
Google Login Page