#backtotop {
/*  background: url(images/backtotop.png) no-repeat center center;*/ 
  bottom: 20px;
  cursor: pointer;
  display: none;
  height: 40px;
  position: fixed;
  right: 20px;
  text-indent:-9999px;
  width: 40px;
  z-index: 1;
  background-color: rgba(56, 67, 89, 0.85);
}
#backtotop::before {
  content: "";
  position: absolute;
  border-left: 2px solid white;
  border-top: 2px solid white;
  width: 15px;
  height: 15px;
  display: inline-block;
  left: calc(50% - 8.5px);
  top: calc(50% - 5px);
  transform: rotate(45deg);
}

#backtotop:hover {
  opacity: 0.8;
}

<!--[if lte IE 6]>
html {
  overflow-x: auto;
  overflow-y: hidden;
}

* html #backtotop {
  position:absolute;
}
<![endif]-->

@media (-webkit-min-device-pixel-ratio: 2),
(min-resolution: 192dpi) {

  #backtotop {
/*    background: url(images/backtotop2x.png) no-repeat center center;
    background-size: 70px 70px;*/
  }

}