.back-to-top {
  display: block;
}
.back-to-top.affix-top {
  opacity: 0;
  transition: all 0.25s ease-out;
}
.back-to-top.affix {
  display: block;
  position: fixed;
  opacity: 1;
  right: 32px;
  bottom: 32px;
}
.back-to-top .btn {
  border: 0;
}
.back-to-top span.fa {
  font-size: 24px;
}