body{
  background: rgb(210, 210, 210);
  transition: background-color 0.5s ease-out;
}
.board_container{
  position: fixed; /* or absolute */
  top: 50%;
  left: 50%;
  margin-top: -250px;
  margin-left: -250px;
  width:500px;
  height:500px;
  background-size : 100% 100%;

}
.board_row{

}
.board_cell{
  width: 50px;
  height: 50px;
  float: left;
  transition: background-color 0s ease-out;
  }
.ball{
  position: absolute;
     width: 20px;
    height: 20px;
    border-radius: 50%;
    -moz-border-radius: 10px;
    -webkit-border-radius: 10px;
    background: red;
  transition: background-color 0.5s ease-out;
}