body:before {
    content: ' ';
    display: block;
    background-color: purple;
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    height: 8px;
    z-index: 10;
}

body:after {
    content: ' ';
    display: block;
    background-color: purple;
    position: fixed;
    top: 0;
    left: 0;
    bottom: 0;
    width: 8px;
    z-index: 10;
}