Edit in JSFiddle

<div id="outer">
  <div id="inner"></div>
</div>

#inner {
  margin: auto;
  width: 250px;
  height: 125px;
  background-image: linear-gradient(45deg, #84ECEF 10%, #F8F62F 60%, #FDC018);
}

#outer {
  height: 500px;
  width: 500px;
  background: #1F1D20;
  background-image: linear-gradient(#757575 1px, transparent 1px), linear-gradient(90deg, #757575 1px, transparent 1px);
  background-size: 25px 25px;
}