.flex-container {
  padding: 0;
  margin: 0;
  list-style: none;
  background:#B1B1B1;
  height:600px;
  display: -webkit-box;
  display: -moz-box;
  display: -ms-flexbox;
  display: -webkit-flex;
  display: flex;
  
  -webkit-flex-flow: row wrap; /* Safari 6.1+ */
  -webkit-align-content: space-around; /* Safari 7.0+ */
  
  align-content: space-around; /*flex-start | flex-end | center | space-between | space-around | stretch;*/
}


  


.flex-item {
  background: tomato;
  padding: 5px;
  width: 200px;
  height: 150px;
  margin: 10px;
  line-height: 150px;
  color: white;
  font-weight: bold;
  font-size: 3em;
  text-align: center;
}