At various points throughout the book, we covered the use of themes within jQuery UI—a useful tool you can incorporate into your pages, is the Super Theme Switcher (STS) plugin, available from https://github.com/harborhoffer/Super-Theme-Switcher. Once implemented, you can use it to switch between themes at will; it's a great way to see how your site's widgets will look when a theme is changed, before making that change permanent.
This is particularly useful if you need to edit an existing theme; it is better practice to place your edits into an override file, and call this separately; editing the core UI CSS files means that you may face issues when upgrading to the next version of jQuery UI, as customizations could be lost.