a.back-to-top {
  display: block; position: fixed; bottom: 2%; right: 2%; width: 37px; height: 37px;
  text-indent: -10000px;
  background: rgba(0,0,0,0.2) url(icon-back-to-top.png) no-repeat center center;
  border: none;
}
a.back-to-top:hover {
  background-color: #fff;
}