Switch theme on specific page

switchtheme.php

<?php
/*
Plugin Name: Switch Theme
Plugin URI:
Description:
Version:
Author:
Author URI:
License:
*/

if (is_admin()){

	/* load plugin functions */
	add_action('add_meta_boxes', 'myplugin_add_meta_box');
	add_action('save_post', 'myplugin_save_meta_box');

} else {

	add_action( 'init', 'my_template_hooks', 5, 0 );

}

	function my_template_hooks(){

		/* load page theme */
		add_filter( 'option_template', 'my_plugin_get_page_theme');
		add_filter( 'template', 'my_plugin_get_page_theme');
		add_filter( 'option_stylesheet', 'my_plugin_get_page_theme');

	}

	/* add meta box to page editing */
	function myplugin_add_meta_box() {

		add_meta_box(
			'',
			'Page Theme',
			'myplugin_get_meta_box_contents',
			'page',
			'normal',
			'default',
			''
		);

	}

	/* meta box contents */
	function myplugin_get_meta_box_contents($post) {

		$themes = wp_get_themes();

		$page_theme = get_post_meta($post->ID, 'page_theme', true);

		echo '<p><label for=page_theme><b>Themes:</b></label></p>';
		echo '<select name=page_theme>';
		echo '<option>(ohne)</option>';

		foreach($themes as $key => $theme) :

			$selected = ($themes[$key]["Template"] == $page_theme) ? 'selected' : '';
			echo '<option value="' . $themes[$key]['Template']. '"'. $selected .'>' . $themes[ $key ]['Name'] . ' </option>';

		endforeach;

		echo '</select>';

	}

	 /* save theme options */
	function myplugin_save_meta_box($post_id) {

		if( isset( $_POST['page_theme'] ) && ! empty( $_POST['page_theme'] ) )
			update_post_meta($post_id, 'page_theme', $_POST['page_theme']);

	}

	function my_plugin_get_page_theme() {

		$page_id = get_the_ID();

		$page_theme = get_post_meta($page_id, 'page_theme', true);
		return $page_theme;

	}