document.getElementsByTagName("head")[0].insertAdjacentHTML( "beforeend", "<link rel=\"stylesheet\" href=\"path/to/style.css\" />");
var cssFile = document.createElement('link'); cssLink1.rel = 'stylesheet'; cssLink1.href = "styles.css"; // or path for file {themes('/styles/mobile.css')} document.head.appendChild(cssFile); // append css to head element
document.querySelector('head').innerHTML += '<link rel="stylesheet" href="styles.css" type="text/css"/>';