Just like Firefox, Thunderbird can be customized with themes that give the email client a fresh look. Themes are installed just like add-ons in Thunderbird by opening the Tools > Add-Ons menu in the Thunderbird top menu and switching to the Themes tab on top. The install button is located on the lower left of that window. A file browser window opens that filters .jar files, the theme format Thunderbird themes come in.
The email client has to be closed and opened again before the new theme becomes visible. It also has to be activated from the themes tab in the add-ons menu before it becomes visible. The easiest way to accomplish the whole process is to install the theme, switch to the theme tab, activate the new theme and restart the browser then.
Many of the themes for Thunderbird at the official Mozilla Thunderbird website do not work with the latest version of Thunderbird which is surprising to say the least. It is probably possible to force compatibility but that has not been tested. All themes listed in this article are working with the latest version of Thunderbird which is currently 2.0.16. They probably will also work with Thunderbird 3.
Update: The installation process of themes in Thunderbird has been changed recently. You know need to click on Tools > Addons to open the email client's add-ons manager. Click on the small settings button in the top right corner next to the search form, and select install add-on from file from the selection menu.
Browse to the downloaded theme to start the installation. You need to allow the installation first, and restart the client afterwards to complete it.
Also note that some of the themes won't work in newer versions of Thunderbird. We have marked those specifically so that you know when this is the case.
ACE CuteBird (Graphite) (not compatible)
Azerty 'mail (not compatible)
Modern Modoki (not compatible)
CrossOver (not compatible)
Simple Green (not compatible)
Noia 2.0 (not compatible)
PitchDark (not compatible)
Here are several themes that work with the most recent version of Mozilla Thunderbird.
Additional themes are available at the themes repository at Mozilla.com.