Short answer: you
can't.
If you read carefully [link]
http://www.dokuwiki.org/plugin:indexmenu[/link]:
Option Action
js Javascript render method: the index is an expandable tree menu.
The un/folding graphical feature is a result of the JS work done behind curtains, so you're out of luck in this one. However, I don't get what is what you don't like: is it the Javascript
theme? it can be changed, check the plugin page.