see full article @ Thunderbird:Themes:Supported/Windows
Window uses the qute theme in Thunderbird.
Also, within the platform are ways to detect if the Windows user is using the default theme via CSS. This allows for a theme that can fall back on regular system colors for accessibility.
The Vista Aero theme requires different icons and colors from the XP theme therefore Thunderbird is using a similar approach as Firefox to create a separate theme space for Vista.
Windows Vista theme tracking bug 488060
Most of the Windows XP theme will be using the Linux Tango style icons and colors. These icons and colors are being reused on Windows XP because they are a similar style (tango) that will allow us to give a good refresh to the Windows XP theme while not spending the majority of our time on Vista icons that will be more relevant in the future.
Windows XP theme tracking bug 488061
see full article @ Thunderbird:Themes:Supported/Linux
Linux distros use the gnomestripe theme provided by Thunderbird.
Currently there is no support for a fall back Thunderbird theme such that icons could be supplied by the OS if they exist or by the gnomestripe theme if they do not exist.
There is a way to create GNOME Theme Application Specific Icons which needs investigation.
Linux Tango icon theme tracking bug 415415
The Mac uses the pinstripe theme within Thunderbird.
The Mac mozilla platform allows for us to detect if the person is using the default mac theme within CSS. This allows for a theme that can fall back on regular system colors for accessibility.