.btn-up {
  position: fixed;
  background-color: lightgrey;
  right: 20px;
  bottom: 20px;
  border-radius: 22px;
  cursor: pointer;
  width: 44px;
  height: 44px;
  display: none;
}

.btn-up::before {
  content: "";
  text-align: center;
  position: absolute;
  width: 20px;
  height: 20px;
  left: 12px;
  top: 12px;
  background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='20' height='20' viewBox='0 0 20 20'%3E%3Cg fill='none' stroke='%23fff' stroke-width='2' stroke-linecap='round' stroke-linejoin='round'%3E%3Cpath d='M10 17V4M3 10l7-7 7 7'/%3E%3C/g%3E%3C/svg%3E");
}

@media (hover: hover) and (pointer: fine) {
  .btn-up:hover {
    background-color: grey; /* цвет заднего фона при наведении */
  }
}