wp_login_form display no styled form

The form appeared un-styled because the login_enqueue_scripts hook does not run on the front-end/non-admin side ( unless you do do_action( 'login_enqueue_scripts' ) in your template ).

So try using wp_head instead: add_action('wp_head', 'custom_login_form');.

You should also add the custom CSS only on the app page, or when the template being used is app.php. For example, you can add this check at the top in your custom_login_form function:

// Do nothing if the template is not app.php.
// This will work the file is at wp-content/themes/your-theme/app.php.
if ( ! is_page_template( 'app.php' ) ) {
    return;
}