JSFiddle - React, Tailwind, and code Playground

HTML

<span id="color-map"></span>

CSS

body {
    background: red;
}

JavaScript

var element = document.getElementsByTagName("body")[0];
var style = window.getComputedStyle(element);

document.getElementById('color-map').innerHTML = rgbsToHex(style.backgroundColor);

function rgbsToHex(str) {
    var hex = "#";
    var matches = str.match(/rgb\((\d+),\s(\d+),\s(\d+)\)/);
    hex += Number(matches[1]).toString(16);
    hex += Number(matches[2]).toString(16);
    hex += Number(matches[3]).toString(16);
    return hex;
}