JAVASCRIPT

javascript add css file

var cssFile = document.createElement('link');
    cssFile.rel = 'stylesheet';
    cssFile.href = "styles.css";  // or path for file {themes('/styles/mobile.css')}
    document.head.appendChild(cssFile); // append css to head element

append css file with javascript

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