#back-to-top {
  position: fixed;
  bottom: 30px;
  right: 30px;
  z-index: 9999;
}

#btnBackToTop {
  background-color: #333;
    color: #fff;
    border: solid #fff 1px;
    border-radius: 50%;
    width: 45px;
    height: 45px;
    font-size: 20px;
    cursor: pointer;
    opacity: 0;
    visibility: hidden;
    transition: opacity 0.3s, visibility 0.3s, background-color 0.2s;
}

#btnBackToTop::before{
  content: "";
  position: absolute;
  left: 50%;
  top: 50%;
  width: 10px;
  height: 10px;
  border-left: 2px solid white;
  border-top: 2px solid white;
  transform: translate(-50%, -50%) rotate(45deg);
}
#btnBackToTop:hover {
  background-color: #555;
}

#btnBackToTop.visible {
  opacity: 1;
  visibility: visible;
}