popup.css

.popup {
  position:fixed;
  color: #4c3327;
  font-family: Helvetica, Arial, sans-serif;
  top:0;
  left:0;
  width:100%;
  height:100%;
  max-width:100%;
  max-height:100%;
  background:rgba(0,0,0, 0.8);
  z-index:3;
  overflow:auto;
  display:none;
}

.popup h2 {
  font-size: 22px;
}

.modal {
  background:#ffffff;
  position:absolute;
  top:50px;
  left:50%;
  margin-left:-350px;
  width:700px;
  padding:20px;
  box-sizing:border-box;
}

.modal .titlebar {
  height:12px;
  margin-bottom:10px;
}

.modal .close {
  position:absolute;
  top:20px;
  right:20px;
  background:url("http://f.cl.ly/items/1n040x241h0w0Q1h020M/close.png") no-repeat;
  background-position:0 0;
  width:12px;
  height:12px;
}

.modal .close:hover {
  background-position:0 -12px;
}

popup.html

<div class="popup" style="display: block;">
  <div class="modal">
    <div class="titlebar">
      <a href="#" class="close" role="button"></a>
    </div>
    <div class="content">
     Swiggity Swag
    </div>
  </div>
</div>