Adding Styles through JS

injectStyle.js

let css = `h1{ font-size : 14px;}`;
let head = document.head || document.getElementsByTagName('head')[0];
let style = document.createElement('style');
style.type = 'text/css';
if (style.styleSheet) {
    // This is required for IE8 and below.
    style.styleSheet.cssText = css;
} else {
    style.appendChild(document.createTextNode(css));
}
head.appendChild(style);