If I can suggest a small change: perhaps you would consider making the
page (leaf node) icon a bit more like other implementations in that
the foldover is at 90 degrees to the page (see attached example)
instead of at an artistic jaunty angle. A big hurdle we have to
overcome in free software is achieving consistency with user
expectations...
The icons have been coded (in the MetalIconFactory class) to match the
appearance of the icons in Swing's Metal look and feel. I'd argue that
that is consistent with user expectations.