demo.html

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Subtle Pattern Demo</title>
<style>
body , html { padding:0;margin:0;height:100%; }
body { background:#ccc none repeat 0 0; }
form { padding:10px;text-align:center;background:#FFF;border-bottom:1px solid #000; }
</style>
</head>
<body>
<form>
<select id="image">
  <option selected>Downloading Pattern Names...</option>
</select>
</form>

<script>
var loadPatterns = function () {
  var ajax, ajaxTimeout, requrl;
  ajax = (window.ActiveXObject) ? new ActiveXObject("Microsoft.XMLHTTP") : (XMLHttpRequest && new XMLHttpRequest()) || null;
  
  ajaxTimeout = window.setTimeout(function () {
    ajax.abort();
  }, 6000);
  
  ajax.onreadystatechange = function () {
    var selectEle, itemsArray, itemsString, i;
    if (ajax.readyState === 4) {
      if (ajax.status === 200) {
        clearTimeout(ajaxTimeout);
        if (ajax.status !== 200) {
          
        } else {
          selectEle = document.getElementById('image');
          itemsArray = JSON.parse(ajax.responseText);
          itemsString = '';
          for(i = 0; i < itemsArray.length; i++) {
            if(itemsArray[i].name.indexOf('.png') !== -1) {
              itemsString += '<option value="'+itemsArray[i].html_url.replace("blob", "raw")+'">'+itemsArray[i].name+'</option>';
            }
          }
          selectEle.innerHTML = itemsString;
          selectEle.setAttribute("onchange", "setPattern(this.value);"); // = setPattern(selectEle.value);
        }
      }
    }
  };
  
  requrl = 'https://api.github.com/repos/subtlepatterns/SubtlePatterns/contents';
  ajax.open("GET", requrl, true);
  ajax.send();
}

var setPattern = function (url) {
  var bodyEle = document.getElementsByTagName('body')[0];
  bodyEle.style.backgroundImage = 'url("'+url+'")';
}

window.onload = loadPatterns();
</script>
</body>
</html>