Using glyphicon as background image in CSS: rails 4 + bootstrap 3

Question

I'm using bootstrap's collapse.js to expand and collapse some input groups in a rails app. I'm using JS to determine if the group is expanded or not and have created CSS classes to add a "+" or "-" to show whether it's open or closed:

Open:

Closed:

As you can see from the CSS, I'm using a background image that's a png within my images:

.expandable {
  background: url('/assets/plus.png');
  padding-top: 4px;
  width: 400px;
  height: 30px;
  background-repeat: no-repeat;
  padding-left: 55px;
  display: block;
}

.expanded {
  background: url('/assets/minus.png');
  padding-top: 4px;
  width: 400px;
  height: 30px;
  background-repeat: no-repeat;
  padding-left: 55px;
  display: block;
}

I would like to use the glyphicon-plus and glyphicon-minus instead of these .png files.

What's the best way to accomplish this?

Updated:

In order to get the proper styling, I changed the CSS to:

.expandable {
  height:40px;
  width:50%;
  margin:6px;

}

.expandable:before{
  content:"\2b";
  font-family:"Glyphicons Halflings";
  line-height:1;
  margin:5px;

}

.expanded {
  height:40px;
  width:50%;
  margin:6px;
}

.expanded:before{
  content:"\2212";
  font-family:"Glyphicons Halflings";
  line-height:1;
  margin:5px;

}

And for reference, my HTML is:

<div class="panel-group" id="accordion">
  <div class="panel panel-default">
    <div class="panel-heading">
      <p1 class="panel-title" >
        <a data-toggle="collapse" data-parent="#accordion" href="#collapseOne" class="expandable">
          Provider details 
        </a>
      <p2>
    </div>
    <div id="collapseOne" class="panel-collapse collapse">
      <div class="panel-body">
        blah, blah....

And JS to detect the opening/closing of the sections:

$ ->
  $(".expandable").click () ->
    $this = $(this)
    if $this.hasClass("expandable")
      $this.removeClass("expandable").addClass "expanded"
    else $this.removeClass("expanded").addClass "expandable"  if $this.hasClass("expanded")
    return
  return

Answer 1

Here, try this Bootply. Would this suffice?

<div id="lol"></div>

---

#lol{
    height:40px;
    border:1px solid #555;
  width:50%;
  margin:30px;
}
#lol:before{
  content:"\2b";
  font-family:"Glyphicons Halflings";
  line-height:1;
  margin:10px;
  display:inline-block;
}