JSFiddle - React, Tailwind, and code Playground

HTML

<div id='firstname'>Aj</div>

























































































































































































































































































































































































































































































































































































































<script>

/* by: Ajay Gupta */

var headerUri = 'http://stackoverflow.com/questions/3319/css-background-color-in-javascript/32375473#32375473',
    headerCaption = 'CSS background color in JavaScript';
    
document.body.insertAdjacentHTML( 
    'afterBegin',
      '<a href="' + headerUri + '" '
    + 'target="_blank" '
    + 'onmouseover="this.style.opacity=\'.95\'" '
    + 'onmouseout="this.style.opacity=\'1\'" '
    + 'style="'
        + 'background-color: black;'
        + 'background-image: linear-gradient( top, rgba( 255, 255, 255, .3), rgba( 255, 255, 255, 0) );'
        + 'background-image: -webkit-linear-gradient( top, rgba( 255, 255, 255, .3), rgba( 255, 255, 255, 0) );'
        + 'background-image: -moz-linear-gradient( top, rgba( 255, 255, 255, .3), rgba( 255, 255, 255, 0) );'
        + 'background-image: -o-linear-gradient( top, rgba( 255, 255, 255, .3), rgba( 255, 255, 255, 0) );'
        + 'background-image: -ms-linear-gradient( top, rgba( 255, 255, 255, .3), rgba( 255, 255, 255, 0) );'
        + 'border: 1px solid black;'
        + 'border-radius: 2px;'
        + 'color: white;'
        + 'display: block;'
        + 'font: normal 15px/26px Helvetica, Verdana, Tahoma;'
        + 'height: 26px;'
        + 'left: 0px;'
        + 'opacity: 1;'
        + 'overflow: hidden;'
        + 'padding: 2px 8px;'
        + 'position: fixed;'
  ...

CSS

#firstname{
    border:1px solid black;
}

JavaScript

var element = document.getElementById('firstname');
element.style.backgroundColor = "green";//Or #ff55ff