.scroll-top-btn{
  position:fixed;
  right:16px;
  bottom:16px;
  width:44px;
  height:44px;
  border-radius:999px;
  border:none;
  background:#111;
  color:#fff;
  font-size:18px;
  font-weight:900;
  display:flex;
  align-items:center;
  justify-content:center;
  cursor:pointer;
  box-shadow:0 6px 16px rgba(0,0,0,.2);
  opacity:0;
  transform:translateY(10px);
  transition:all .25s ease;
  z-index:999;
}

.scroll-top-btn.is-show{
  opacity:1;
  transform:translateY(0);
}

.scroll-top-btn:hover{
  background:#333;
}