I'm running Thunderbird 8 on Ubuntu 11.10, and I'm stuck with these monochrome toolbar buttons that I hate. If I look under
Tools > Add Ons > Appearance I've only got the default theme installed and the sample icons are full color. The icons I'm seeing are decidedly not full color. I was able to install the Gnome Tweak Tool which let met change the rest of my Unity icons to full color by switching the "icon theme" to "Unity-icon-theme" (it was Unity-mono-dark). Now only my Thunderbird icons are monochrome.
How is Unity overriding the default Thunderbird theme?