Martin Picek's DancingMouse theme with menu icons that plays nice with Thunderbird3.
Thunderbird 3.0a1pre - 3.0b1pre
Experimenting before resulted in a lot of useless files. I just realized that the past versions (in which I tried to prune the useless files) were broken. This time I was on the side of caution and left a lot of files in there, but at least it works.