Edit in JSFiddle

<a id='backColor' href="#">Back color changes</a>
body {
  font-family: Calibri;
  font-size: 16px;
}

#backColor {
  text-decoration: none;
  padding: 0 5px;
  transition: all .9s;
}

#backColor:hover {
  color: #fff;
  background-color: green;
}