<p>Two ways to horizontally center blocks that are absolute positioned</p>

<div class="container">
  <div class="lightbox-1">
    Good when you need old browser support, must know lightbox width
  </div>
</div>

<div class="container">
  <div class="lightbox-2">
    Good when you don't need older browser support whether
    width is known or not.
  </div>
</div>

.container {
  background-color: #CCC;
  position: relative;
  height: 120px;
  margin: 10px;
}

.lightbox-1 {
  position: absolute;
  width: 400px;
  height: 100px;
  background-color: red;
  top: 10px;
  
  /* 
  Centering
  */
  left: 50%;
  margin-left: -200px;
}

.lightbox-2 {
  position: absolute;
  width: 400px;
  height: 100px;
  background-color: green;
  top: 10px;
  
  /* 
  Centering
  */
  transform: translateX(-50%);
  left: 50%;
}