it isn't only good, but there is a script called change.sh, what is it?
simple, it is a script, to change the default icon, try to understand.
I thought about create a gui interface at this script, to become easy to customize all icon set, look, is'nt difficult.
The author have only to create an alternative at the deafult set.
So, if I understood your post correct; you want an application where you can edit an existing icon theme?
My proposal would be that this editor creates an empty theme and lets it inherit icons from a selected source, in this case you can edit themes without needing root access (for icons in /usr/share/icons) and without destroying the original theme.