If you have this in your CSS file:
.myApp
{
border-bottom-color:#FF0000;
}
and a div for instance of:
<div id="myDiv">test text</div>
you can use:
$("#myDiv").addClass('myApp');// to add the style
$("#myDiv").removeClass('myApp');// to remove the style
or you can just use
$("#myDiv").css( 'border-bottom-color','#FF0000');
I prefer the first example, keeping all the CSS related items in the CSS files.