body {
  margin: 20px auto;
  width: calc(100% - 20px);
}

@media screen and (min-width: 769px) {
  body { width: 750px; }
}

@media screen and (min-width: 900px) {
  body { width: 850px; }
}

@media screen and (min-width: 1100px) {
  body { width: 1000px; }
}