I am using Mozilla Thunderbird from a very long time, its very simple and easy to use, I tried many other email clients but sometime they required manually adding servers name like many times for Google Apps emails.
Thunderbird adds most of the servers itself and required very less work from your side, As I am using it from a very long time but I never tried to customize it but today I think to change its theme as I was bored of seeing that classic look and found a awesome Mac Leopard theme,
The theme is very cool and you will feel that you are using a Mac application or running OS.
How To Install :-
Installation is very easy all you need to do is download them from below link and then open your Thunderbird then click on Tools –> Add-On and then a pop up dialog box comes, then click on Theme tab , Now click on Install select the location and theme which you downloaded and install it.
It will required restart of Thunderbird.
You may use following link to download it :-