Using custom tree expand/collapse icon

[Causevic, Dzenan]

 Is it possible to use your own custom tree expand/collapse icon that user 
clicks on to expand or collapse a subtree?

This is the excerpt from my css file regarding tree components. As you can see 
below I am trying to specify background-image that would supposedly overrun the 
default one, but I dont get any success, the default one is still showing thru 
even if I try to set it to none (background-image: none;)

/* tree */
/* -------------- */
.p_OraTreeDisclosedSymbol {
   border-right: lightcyan solid 0.5px;
   border-bottom: lightcyan solid 0.5px;
   background-image: url(/images/showarrow.gif);
}

.p_OraTreeNodeAdjust {
   font-size: 7pt;
}

.p_OraTreeRow {
   background-color: none;   
}