theme_page_test.module

<?php


/**
 * @file
 * Test module.
 */
use Drupal\Core\Extension\Extension;

/**
 * Implements hook_system_info_alter().
 */
function theme_page_test_system_info_alter(&$info, Extension $file, $type) {
    // Make sure that all themes are visible on the Appearance form.
    if ($type == 'theme') {
        unset($info['hidden']);
    }
}