Edit in JSFiddle

JavaScript
HTML
SCSS
Result

<body>
  <h1>Prashant Says hello! - <a href="https://www.prashant.me/development/2016/01/03/rainbow-border-with-linear-gradient-in-s-css.html">prashant.me</a></h1>
</body>

$gradient_ : 90deg, #1abc9c 15%, #2ecc71 15%, #2ecc71 12%, #3498db 12%, #3498db 32%, #9b59b6 32%, #9b59b6 35%, #34495e 35%, #34495e 55%, #f1c40f 55%, #f1c40f 59%, #e67e22 59%, #e67e22 63%, #e74c3c 63%, #e74c3c 82%, #ecf0f1 82%, #ecf0f1 92%, #95a5a6 92%;

$size_  : 100% 7px;

body{  
    background-image: -webkit-linear-gradient($gradient_);
    background-image: -khtml-linear-gradient($gradient_);
    background-image: -moz-linear-gradient($gradient_);
    background-image: -o-linear-gradient($gradient_);
    background-image: linear-gradient($gradient_);

    background-repeat: no-repeat;
    background-position-y: top;

    -webkit-background-size: $size_;
    -khtml-background-size: $size_;
    -moz-background-size: $size_;
    -o-background-size: $size_;
    background-size: $size_;

    h1{
        text-align: center;
        margin: 20px 0 0 0;
    }
}