Only tested on ubuntu 10.04.4 with natty's ambiance theme but is should work with all system themes (some thunderbird themes may have a conflict or 2 )

Click for full size which is 1920x1080

http://i.imgur.com/Zg6PY.png

http://i.imgur.com/zijEu.png

If you don't want to install stylish in Thunderbird you can use the userChrome.css file
Cresting a userChrome.css file in Linux from the terminal (This will probably work on a mac also)

mkdir ~/.thunderbird/`ls ~/.thunderbird/ | grep .default`/chrome

nano ~/.thunderbird/`ls ~/.thunderbird/ | grep .default`/chrome/userChrome.css

then you just paste the css code in and press [ctrl]+[O] then [Enter] then [ctrl]+[X]
if that is too complicated change nano to gedit/mousepad/kedit/geany/scite or what ever text editor you like to use

in windows you will need to go to start -> run
start %appdata%
navigate to a folder called Thunderbird and into a colder called [anything.default] create a colder called chrome and make a new file called userChrome.css