#left-bar{float:left;width:250px;padding-bottom:20px;position:relative;z-index:2;}
#center-bar{padding-bottom:20px;margin-left:300px;min-height:600px;}
#left-bar{display:none}
#center-bar{margin-left:0;}
@media (min-width: 768px){
    #left-bar{display:block}
    #center-bar{margin-left:300px;}
}