<div><span></span></div>

body {
  background: #505050;
}
div {
    position: relative;
    width: 100px;
    height: 100px;
    margin: 20px;
    background: #023677;
}
div:after {
    display: block;
    content: "";
    width: 20px;
    height: 35px;
    position: absolute;
    top: -3px;
    right: -3px;
    border-top: 3px solid yellow;
    border-right: 3px solid yellow;
}
span:before {
    display: block;
    content: "";
    width: 20px;
    height: 35px;
    position: absolute;
    bottom: -3px;
    left: -3px;
    border-bottom: 3px solid yellow;
    border-left: 3px solid yellow;
}