****UPDATE***

Finally figured out how to customize expanders in a Pixmap theme, which I've never seen done.  Here is the code if anyone wants to give it a try.  EXPANDER CODE

Make an Expanders folder in your gtk theme and put 3 images in there that you want for the expanders, since its kind of animated there will be an expanded, semi-expanded and collapsed.  Semi-expanded and semi-collapsed use the same image of course.  I have +,- and one in between for the images as example.

End Update