#outer-circle {
  background: #385a94;
  border-radius: 50%;
  height: 500px;
  width: 500px;
  position: relative;
  /*
   Child elements with absolute positioning will be
   positioned relative to this div
  */
}
#inner-circle {
  position: absolute;
  background: #a9aaab;
  border-radius: 50%;
  height: 300px;
  width: 300px;
  /*
   Put top edge and left edge in the center
  */
  top: 50%;
  left: 50%;
  margin: -150px 0px 0px -150px;
  /*
   Offset the position correctly with
   minus half of the width and minus half of the height
  */
}