bullet list question

Great plugin !

Your images (+ and -) for the list bullets don’t suit my site because my background is too dark. So for the moment, I’m using the plugin’s ? and ▼ characters but I’d prefer my own images.

I’ve tried adding code in my theme’s css but it’s not working although i’ve added other pieces of codes for changing the widget style and those are working.
As bullets, I would like
– closed folder icon for default li or li (class .expand)
– open folder icon for li (class .collapse)
– no icon for the end li’s (articles) (class . item)

The result however is the closed folder icon on every li. Also on the end li (.item) for which I want no icon. I checked the images folder of my theme. Both images are in it.
Here’s a screecap of it : https://i6.photobucket.com/albums/y219/francoise/galery_tree_screencap_zpsd83c6c31.jpg

In the widget settings, I checked “custom” and entered no character.
Here is my css addition concerning the bullets :

li.collapsing.categories {
  background: transparent url("images/folder_open2_h11.png") no-repeat scroll 0px 3px !important;
}
li.collapsing.categories.expand {
  background: transparent url("images/folder_closed2_h11.png") no-repeat scroll 0px 3px !important;

li.collapsing.categories.collapse {
  background: transparent url("images/folder_open2_h11.png") no-repeat scroll 0px 3px !important;
}

li.collapsing.categories.item {
  background: transparent url("none") no-repeat scroll left top !important;

I’ve tried to no specify the background for the default li.collapsing.categories (since it’s specified for li….expand but then no icon at all is showing.

If I could get some help, I’d be happy.
Thanks in advance, Fran

https://www.ads-software.com/extend/plugins/collapsing-categories/