OK, my bad. I had trouble grabbing the window border in the Tools --> Add-ons --> Themes. All OK in FF.
I then tried the current Thunderbird schemes -
Mac4Lin_1.0_RC2_TB_Aqua.jarAll the command buttons have a vertical white bar running through the left side of the button. The font seems too small and don't center in the command buttons.
Compare Thunderbird (left) to FF (right). 1900 x 1200 resolution. Thunderbird version 18.104.22.168 (20090409). (Window border is MacOS-X from www.gnome-look.org- backup theme when Mac4Lin is broken.)