body {
  background-color: #eee;
}

.section {
  margin: 30px;
  position: relative;
  background-color: #03a9f4;
  width: 400px;
  height: 300px;
  
  &:before, &:after {
    content: '';
    display: block;
    width: calc(100% + 10px);
    height: calc(100% + 10px);
    position: absolute;
    background-color: rgba(#03a9f4, .5);
  }
  
  &:before {
    right: -10px;
    top: -10px;
  }
  
  &:after {
    left: -10px;
    bottom: -10px;
  }
}