Although Mozilla Thunderbird is an incredibly lean mail client, it is by no means bare-bones. And owing to the seriousness of the Mozilla team, the application looks and feels very slick. And although the default theme for Thunderbird is pretty attractive, enthusiasts have gone ahead and created some true masterpieces to skin Thunderbird. Whether you want large icons, dark backgrounds, pink colored toolbars, Apple-flavored themes or a very basic look, you will surely find something made for your need.
Installing these themes is pretty easy too. Just make your way to the Thunderbird addons site and select the Themes section. Once you've browsed around and found the theme you're looking for, just download it and save it. Now just open the Addons > Themes window from the Tools menu and drag the downloaded .jar file into it. It will install the theme and ask you for a restart of Thunderbird. Just select the theme you want to apply. A final restart is needed to apply that theme.