First, I would like to say that this is a very nice plugin, thank you to the author. While I like it, it is hard to use custom css with, especially for a non-guru. Not sure why exactly but I can't get it to display the folders at all just the bullets plus/minus.
Add the option to close other nodes so that only one node per level is open at a time.
My main suggestion would be to have a default css which you do, and then allow users to override the default settings in the options with their own open/close icons and to be able to turn folder tree on and off in the options rather than users editing the css files.
Thanks for the plugin