You cannot edit css file itself but can change or add css propreties.
Here is a DEMO to help YOU http://jsfiddle.net/Simplybj/EFsvz/2/
You can add muliple css properties to your html like
$('#demo').click(function(){
$(this).css({'background-color':'red','color':'white');
});
Or if you have css Class defined in style sheet like
.makeRed
{
background-color:red;
height:100px;
width:100px;
margin:5px;
}
.makeGreen{
background-color:green;
height:100px;
width:100px;
margin:5px;
}
And Markup like
<div id="me" class="makeRed"></div>
Then You can change class of div as
$('#me').click(function(){
$(this).removeClass('makeRed').addClass('makeGreen');
});