a {
    outline: none; 
}

#back_top{

background:url('http://rebill.me/js2/up_a.png') no-repeat top left;
width:30px;
height:30px;
position:fixed;
/*border-radius:2px;*/
padding:0px;
 _position:absolute;_top:expression(documentElement . scrollTop+documentElement . clientHeight-this . offsetHeight - 150);left:0;bottom:150px;
display:none;
z-index:1000; 
box-shadow: 0 1px 2px 0 rgba(0, 0, 0, 0.3);
}

#back_top:hover{ 
background:url('http://rebill.me/js2/up_na.png') no-repeat top left;
}