.content {
  max-width: 900px;
  margin: auto;
}
       
  
  
  
  #myBtn {
  display: none;
  position: fixed;
  bottom: 40px;
  right: 30px;
  z-index: 99;
  font-size: 30px;
  border: none;
  outline: none;
  background-color: red;
  color: white;
  cursor: pointer;
  padding: 15px;
  border-radius: 4px;
  
}

#myBtn:hover {
  background-color: #4dd2ff;
}


#container:hover > #cube {background:#66d9ff; transition:1s;}
#container1:hover > #cube1 {background:#66d9ff; transition:1s;}
#container2:hover > #cube2 {background:#66d9ff; transition:1s;}