This code will allow you to initiate a theme changer in your site for jQuery UI.

themeselector.js

/*
* Author: Jonathan Gamble
* jQuery UI Theme Changer
*/
  
$(document).ready(function() {
  
  // id of your theme selector <div>
  themeSelector('#themeSelector');

});

function themeSelector(e) {
    
  // Get all themes   

  var themes = [
  'Black Tie',    'Blitzer',        'Cupertino',
  'Dark Hive',    'Dot Luv',        'Eggplant',
  'Excite Bike',  'Flick',          'Hot Sneaks',
  'Humanity',     'Le Frog',        'Mint Choc',
  'Overcast',     'Pepper Grinder', 'Redmond',
  'Smoothness',   'South Street',   'Start',
  'Sunny',        'Swanky Purse',   'Trontastic',
  'UI darkness',  'UI lightness',   'Vader'
  ];
  
  // Source
  
  var jVersion = '1.11.4';
  var google_src = 'https://ajax.googleapis.com/ajax/libs/jqueryui/';
  
  // Create select

  var changer = $('<select>').appendTo(e);
  $(themes).each(function() {
      changer.append($("<option>").attr('value', this.replace(' ', '-').toLowerCase()).text(this));
  });
  
  // Add change event
  
  $(changer).selectmenu({
      change: function() {
          var css = google_src + jVersion + '/themes/' + this.value +'/jquery-ui.min.css';
          $('link[href*="jquery-ui."]').attr('href', css);
      }
  });
    
}