First you need to download and upload the PHP-FB SDK from https://github.com/facebook/php-sdk

Then you can insert the following code into your functions.php

require_once 'path/to/fb/php/sdk/src/facebook.php';

define("FB_APPID", "****YOUR FB APP ID****");

define("FB_SECRET", "****YOUR FB SECRET****");



global $fb, $signedRequest;

$fb = new Facebook(array(

	'appId' => FB_APPID,

	'secret' => FB_SECRET

));

$signedRequest = $fb->getSignedRequest();



function fb_user_theme( $template = '' ) {

	global $fb;

	if (isFacebook()) {

		return "***YOUR THEME NAME***";

	}

	return $template;

}



function isFacebook() {

	global $signedRequest;

	if ($signedRequest != null) {

		return true;

	}

	return false;

}



add_filter('template', 'fb_user_theme');

add_filter('stylesheet', 'fb_user_theme');

Mind you this will change the theme that is loaded. Also note that I have loaded the FB sdk into the global variable $fb and I have loaded the signed request into $signedRequest so that you can reuse them.