body{
  color:#FFF;
  font-family:Helvetica, Arial, sans-serif;
  
}

.header{
  background-color:#232323;
  margin:0;
}

.header,.child{
  padding:20px 30px;
}

.parent{
  display:flex;
  justify-content:space-around;
}

.child{
  width:25%;
  box-sizing:border-box;
}


.child:nth-child(odd){
  background-color:#323232;
}

.child:nth-child(even){
  background-color:#757575;
}

@media all and (max-width:960px) {
  .parent{
    flex-flow: row wrap;
  }  
  .child{
    width:50%;
  }
}

@media all and (max-width:550px) {
  .parent {
     flex-direction:column;
  }
  .child{
    width:100%;
  }
}