Aangaande dié byvoeging
Please note I've never created any theme before. I just extracted relevant parts of the omni.ja file from Thunderbird 12 and packed it up as a theme. It's working for me (TM), but if you find any issues please don't hesitate to contact me!
If this doesn't work for you there is a very simple way - just download stock thunderbird and replace omni.ja file in your thunderbird installation (/usr/lib/thunderbird in *buntu) with the stock one.