.back-to-top__button {
  position: fixed;
  bottom: 20px;
  right: 20px;
  width: 40px;
  height: 40px;
  border-radius: 50%;
  justify-content: center;
  align-items: center;
  cursor: pointer;
  opacity: 0;
  transition: opacity 0.3s;
}

.back-to-top__icon {
  transition: fill 0.3s;
  -webkit-transition: fill 0.3s;
  -moz-transition: fill 0.3s;
  -ms-transition: fill 0.3s;
  -o-transition: fill 0.3s;
}

.back-to-top__show-button {
  opacity: 1;
}

.back-to-top__button:hover .back-to-top__icon {
  fill: darkred;
}