.parent-div {
   position: relative;
   width: 100px;
   height: 100px;
   background: #ffffff;
   border: 1px solid #000000;
}
.div:before {
  content: "";
  width: 20px;
  height: 20px;
  background: blue;
  position: absolute;
  top: 45%;
  left: 45%;
}