1document.getElementsByTagName("head")[0].insertAdjacentHTML(
2    "beforeend",
3    "<link rel=\"stylesheet\" href=\"path/to/style.css\" />");

1var cssId = 'myCss';  // you could encode the css path itself to generate id..
2if (!document.getElementById(cssId))
3{
4    var head  = document.getElementsByTagName('head')[0];
5    var link  = document.createElement('link');
6    link.id   = cssId;
7    link.rel  = 'stylesheet';
8    link.type = 'text/css';
9    link.href = 'http://website.com/css/stylesheet.css';
10    link.media = 'all';
11    head.appendChild(link);
12}
13

1var cssFile = document.createElement('link');
2    cssFile.rel = 'stylesheet';
3    cssFile.href = "styles.css";  // or path for file {themes('/styles/mobile.css')}
4    document.head.appendChild(cssFile); // append css to head element

1var cssFile = document.createElement('link');
2    cssLink1.rel = 'stylesheet';
3    cssLink1.href = "styles.css";  // or path for file {themes('/styles/mobile.css')}
4    document.head.appendChild(cssFile); // append css to head element
5