var links = document.getElementsByTagName("link");
var hasStyleSheet = false;

for (var i = 0; i < links.length; i++) {
  if (links[i].type == "text/css") {
    hasStyleSheet = true;
  }
}

if (!hasStyleSheet) {
  var el = document.getElementsByTagName("body")[0].style;

  el.maxWidth = "40%";
  el.margin = "0 auto";
}