It should be easy to integrate it for HTML and CSS, you'll spend a lot of time to make the layout and then it'll be faster for the rest. However, the version of jQuery used by XenForo could be a problem for JavaScript.
I know someone who's done it, and he said it was a tad difficult. Given enough time, you could definitely get a smooth integration, however you'd need an idea of what you want the template to look like before you start
It should be easy to integrate it for HTML and CSS, you'll spend a lot of time to make the layout and then it'll be faster for the rest. However, the version of jQuery used by XenForo could be a problem for JavaScript.