This add-on doesn't require Stylish or anything else. I tested it on
64bit system and it works fine. But you wrote you were using 64bit
version of Thunderbird. Did you download it from here:
http://wiki.mozilla-x86-64.com ? I can't test it right now, but this can
be the reason. Also, could you write what add-ons you have installed and
which OS you use (Vista/7)?