<?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');

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

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

    }

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

        $themes = 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) {

        update_post_meta($post_id, 'page_theme', $_POST['page_theme']);

    }

endif;

/* 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');

function my_plugin_get_page_theme() {

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

}

?>