There are several issues with your code. Let’s start at the beginning…
Adding hooks
All your hooks will be added at the backend. Even those which are outside the if((is_admin)) branch. Add your hooks explicitly where they should work:
if (is_admin()){
// do this on backend (admin side)
/* load plugin functions */
add_action('add_meta_boxes', 'myplugin_add_meta_box');
add_action('save_post', 'myplugin_save_meta_box');
} else {
// if we are NOT on backend (admin side) we have to be on frontend, because there is only frontend and backend.
/* 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');
}
In the loop or not?
get_the_ID() must be within the loop. Where are your hooks (‘option_template’, ‘template’ and ‘option_stylesheet’) are done? Before the query is done or after (means: inside the loop or outside) ?
Two lines of code give us the answer:
global $post;
var_dump( $post );
If $post is empty (null), we are outside the loop. We hooked to early. We need a hook after the query is done. The ´init´ hook is an odd one. Let’s alter the code above a bit and get the plugin working:
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');
}
Now our hooks are inside the loop and the plugin works fine.
Deprecated function
On line 28 of your plugin code you use $themes = get_themes();. get_themes() is deprecated and should be replaced with ´wp_get_themes()´.
Development
Please enable WP-debugging (set WP_DEBUG to TRUE in your wp-config.php). The debug messages will help you much.
You can find the complete modified code on Github