html, body {
  background: black;
}

#codepen-link {
  position: absolute;
  bottom: 20px;
  right: 30px;
  height: 40px;
  width: 40px;
  z-index: 10;
  border-radius: 50%;
  box-sizing: border-box;
  background-image: url("https://s3-us-west-2.amazonaws.com/s.cdpn.io/544318/logo.jpg");
  background-position: center center;
  background-size: cover;
  opacity: 0.4;
  transition: all 0.25s;
}
#codepen-link:hover {
  opacity: 0.8;
  box-shadow: 0 0 6px #efefef;
}