style.css

.header {
  background-color: #333333;
  height: 100vh;
}
.section {
  height: 4000px;
  border: 1px solid rebeccapurple;
}
.scroll-up {
  position: fixed;
  right: 50px;
  bottom: 50px;
  z-index: 25;
  width: 50px;
  height: 50px;
  color: aqua;
  cursor: pointer;
  visibility: hidden;
  opacity: 0;
  border-radius: 50px;
  box-shadow: inset 0 0 0 4px aquamarine;
  transition: 0.2s;
  transform: translateY(25px);
}
.scroll-up::before {
  content: '↑';
  position: absolute;
  top: 0;
  left: 0;
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100%;
  width: 100%;
  font-size: 40px;
}
.scroll-up:hover {
  color: orange;
}
.scroll-up--active {
  visibility: visible;
  opacity: 1;
  transform: translateY(0);
}
.scroll-up__svg-path {
  box-sizing: border-box;
  fill: none;
  stroke: orange;
  stroke-width: 4px;
  transition: 0.2s;
}