JSFiddle - React, Tailwind, and code Playground

HTML

<head>
  <link type="text/css" rel="style" href="c:/css/etx_global.css">
</head>

<body>
  <div>
    <buttom class='btn'>Submit1</buttom>
  </div>

<div>
    <buttom class='custom'>Submit2</buttom>
</div>

</body>

CSS

.btn {
  background-color: green;
  /* Green */
  border: none;
  color: white;
  padding: 15px 32px;
  text-align: center;
  text-decoration: none;
  display: inline-block;
  font-size: 16px;
}

.custom .btn {
  background-color: brown;
  /* Green */
  border: none;
  color: white;
  padding: 15px 32px;
  text-align: center;
  text-decoration: none;
  display: inline-block;
  font-size: 16px;
}

JavaScript

var $doc = $doc || document; //make it compatible with GWT
$doc.getElementsByTagName("body")[0].className += " custom";

//alert($doc.getElementsByTagName("body")[0].className);