Client-Side Stylesheet Replacer with jQuery--Simple, not efficient, but works.

html

<select name="theme-menu" id="themes" >
  <option value="override">Default Theme</option>
  <option value="penn-theme">Penn Theme</option>  
  <option value="neutral-theme">Neutral Theme</option>
</select>

js

$( document ).ready(function() {

var theme;

function setTheme(theme){
	switch (theme) {
		case "override":
			$('link[rel=stylesheet][href$="penntheme.css"]').remove();
			$('link[rel=stylesheet][href$="neutral-theme.css"]').remove();
			$('head').append('<link rel="stylesheet" href="assets/design/css/' + theme + '.css" type="text/css" />');
			break;
		case "penn":
			$('link[rel=stylesheet][href$="override.css"]').remove();
			$('link[rel=stylesheet][href$="neutral-theme.css"]').remove();
			$('head').append('<link rel="stylesheet" href="assets/design/css/' + theme + '.css" type="text/css" />');
			break;
		case "neutral":
			$('link[rel=stylesheet][href$="override.css"]').remove();
			$('link[rel=stylesheet][href$="neutral-theme.css"]').remove();
			$('head').append('<link rel="stylesheet" href="assets/design/css/' + theme + '.css" type="text/css" />');
			break;
		}
}

$('select[name="theme-menu"]').change(function() {
	theme = $('#themes').val();
	setTheme(theme);
});