CSS

z index

z-index => allows you to LAYER your components
z-index: 2147483638 is the maximun "height" you can give your component
otherwise it will not work

z-index

z-index: -1;

z-index

/* The z-index property specifies the stack order of an element. */
.z-index {
   z-index: -1;
}

z-index inside z-index

You can do this, but only if the parent doesn't have a 
position: relative or absolute property on it. 
So if you remove the position: absolute from the 
first div, it'll work as expected.

z-index concept

<div class="wrapper">
  <div class="dashed-box">Dashed box</div>
  <div class="gold-box">Gold box</div>
  <div class="green-box">Green box</div>
</div>

ex: z-index

#blue {
  background-color: blue;
  z-index: 3;
}

#red {
  background-color: red;
  z-index: 2;
}

#green {
  background-color: green;
  z-index: 1;
}