* { box-sizing: border-box; }

/*
  = = = = = = = = = =
  Vous pouvez supprimer la suite du code
*/

body.class1 {
  background: #ccc;
}

#item {
  width:800px;
  height:500px;
  position:absolute;
  top:0;
  left:0;
  background:#00ffba;
  z-index:-1;
}

#item.class2 {
  background:#221111;
}

#header {
  margin:30px;
  padding: 10px 30px 30px 30px;
  border: solid 8px black;
  background:white;
  box-shadow: 16px 16px 0 #ff0087;
}