Dynamically loading CSS

I'm trying to create a page theme function for my site. I want to load the corresponding themes dynamically on the page using separate CSS files.

I'm using this code:

  fileref.setAttribute("rel", "stylesheet")
  fileref.setAttribute("type", "text/css")
  fileref.setAttribute("href", 'link.css')

  document.getElementsByTagName("head")[0].appendChild(fileref)

Which works fine, but it doesn't return any info if the CSS file has loaded or not.

Is there a way to catch when the .css is loaded? Maybe by using ajax?