#back-to-top {
  background: #B50000;
  color: #fff;
  position: fixed;
  bottom: 100px;
  right: 30px;
  z-index: 10;
  width: 40px;
  height: 40px;
  text-align: center;  
  cursor: pointer;
  border: 0;
  border-radius: 50%;
  text-decoration: none;
  opacity: 0;
  transition: all 0.25s;
  outline: none !important;
}

#back-to-top:after {
  font-family: "Font Awesome 5 Free";
  content: '\f077';
  font-weight: bold;
  line-height: 36px; /* adjust according icon */
}

#back-to-top:hover {
  background: #333;
}

#back-to-top.show {
  opacity: 1;
}