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

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;
}

Answer 2

I did something similar to get an arrow effect, using absolute positioning to move the glyph to where I wanted it:
li.arrow {
&:after {
content:"\e080";
font-family:"Glyphicons Halflings";
position: absolute;
right: 15px;
top: 10px;
}
padding-right: 25px;
}