Vista Mail offers a theme featuring MS Vista icons.
This is the first theme I've attempted to use, but it was incredibly easy to install. All you need is Thunderbird 1.5-2.0, and you'll be up and running in no time. Simply right-click the link above and select "Save Link As..." to download the theme to your hard disk.
Once you've downloaded the file, open the extension manager in Mozilla Thunderbird (located under the Tools menu), and click the install button. Then find and select the downloaded file, and click "OK" to install. From there, just sit back and enjoy the modern, sleek interface that Vista Mail provides.
Version 1.0: N/A