Edit in JSFiddle

html, body {
  height: 100%;
}

.wrapper {
  background: url(http://www.codingepiphany.com/hotlink-ok/codingepiphany_snow.jpg);
  background-position: top left;
  background-repeat: no-repeat;
  height: 100%;
  width: 100%;
}

.modal {
  position: relative;
  margin: 0 auto;
  top: 35%;
  height: 200px;
  width: 60%;
  overflow: hidden;
  border-radius: 5px;
  background-color: rgba(255, 255, 255, 0.5);
}

.modal .blurrer {
  position: absolute;
  top: 0;
  left: 0;
  background: url(http://www.codingepiphany.com/hotlink-ok/codingepiphany_snow.jpg);
  background-position: top left;
  background-repeat: no-repeat;
  -webkit-filter: blur(5px);
  -moz-filter: blur(5px);
  -o-filter: blur(5px);
  -ms-filter: blur(5px);
  filter: blur(5px);
  z-index: 10;
  height: 200px;
  width: 100%;
}

.modal .message {
  z-index: 20;
  position: relative;
  text-align: center;
  font-family: helvetica, arial, sans-serif;
  color: #fff;
  text-shadow: 2px 2px 2px #888;
  font-size: 1.3em;
  height: 100%;
  display: table;
}

.modal .message .row {
  display: table-cell;
  vertical-align: middle;
  padding: 10px;
}