Hi Jeff,
thanks for picking this up.
To cover the time until such documentation exists, I compiled a script where you can watch the change to your favorite <node3d:GUI> when items of a theme are changed. All you have to do is to rename the script from .txt to a .py ending (py not allowed for upload). Then, create additional GUI elements you want to change, and add them to the oList (object list).
Hope this helps someone else, too.
By the way, Jeff, while you are at it - please also add a documentation of skins - as in <panel>.setSkin(skin)
BR Walter