body {
  background-color: #ee2b7c;
  background: url("PolkaDot@2x.png")  repeat;
  background-size: 141px 138px;
}

#mainContent {
   position: absolute;
   top: 50%;
   left: 50%;
   width: 600px;
   height: 400px;
   margin-top: -200px; /* Half the height */
   margin-left: -300px; /* Half the width */
}