JSFiddle - React, Tailwind, and code Playground

HTML

<div class="colors">
</div>

CSS

.colors {
    position: relative;
    width: 100px;
    height: 50px;
    background: linear-gradient(to right, 
        #78C5D6,
        #459BA8,
        #79C267,
        #C5D647,
        #F5D63D,
        #F08B33,
        #E868A2,
        #BE61A5);
}
.colors::after {
    width: calc(100% - 20px);
    height: calc(100% - 20px);
    position: absolute;
    top: 0;
    left: 0;
    border: 10px solid rgba(0,0,0,0.1);
    z-index: 5;
    content:""
}