[css] Center div on the middle of screen

Your code is correct you just used .div instead of div

HTML

<div class="ui grid container">
<div class="ui center aligned three column grid">
  <div class="column">
  </div>
  <div class="column">
    </div>
  </div>

CSS

div{
position: absolute;
top: 50%;
left: 50%;
margin-top: -50px;
margin-left: -50px;
width: 100px;
height: 100px;
}

Check out this Fiddle