p.ex1 {margin: 10px 28%}
@media (max-width: 767px) {p.ex1 {margin: 10px 5%}}

body {
  background-color: #e0f8d0;
color: #081820;
}

.shelf {
  height: 30px;
  width: 800px;
  background-color: #88c070;
}

<!--- Hey, how the hell did you get in here? Get off my CSS page! --->