Theming Mail.app to look like iPad's Mail.app or Thunderbird
The Mail.app program in Snow Leopard is not easily themable.  

There are 3rd party applications that can change various theme elements (eg http://mac.appstorm.net/how-to/graphics-how-to/an-introduction-to-changing-os-x-themes/) but it is unlikely that even these will get the exact look you are after.  

And most of them will change the entire look and feel of the operating system, not just selected applications.