Edit in JSFiddle

.square {
  width: 100px;
  height: 100px;
  background: #439AE2;
  margin: 20px;
  border-radius: 5px;
  -webkit-transition: background-color 1s ease-out;
  -moz-transition: background-color 1s ease-out;
  -o-transition: background-color 1s ease-out;
  transition: background-color 1s ease-out;
}

.square:hover {
  background-color: #256BA6;
  cursor: pointer;
}

<div class="square"></div>