I had some trouble with that too, since I tried to simply move the icon to the right side.
This is how I adapted the CSS for that span:
.ui-collapsible-heading a span.ui-btn {
position: absolute;
right: 6px;
top: 50%;
margin: -12px 0 0 0;
width: 20px;
height: 20px;
padding: 1px 0px 1px 2px;
text-indent: -9999px;
}
I changed the left to right. If you are designing a custom theme that overwrites the default classes, don't forget to add left:auto to the new style.
Hope it helps!
Mary Lou