a.back-to-top {
  position: absolute;
  z-index: 1000;
  display: none;
  width: 28px;
  height: 28px;
  background-color: white;
  opacity: 0.6;
  filter: alpha(opacity=60);
}
a.back-to-top:hover {
  opacity: 0.9;
  filter: alpha(opacity=90);
}