Check if theme exists and is active or die

theme-guard.php

<?php

/**
 * Plugin Name: Theme Guard
 * Plugin URI: https://kybernaut.cz/?p=3204
 * Description: Check if theme exists and is active or die.
 * Version: 0.0.1
 * Author: Karolína Vyskočilová
 * Author URI: https://kybernaut.cz
 * License: MIT
 */

add_action('init', function () {

    $current = wp_get_theme();
    $web = wp_get_theme('twentytwenty');

    if (!$web->exists() || $current->name !== $web->name) {
        wp_die('An error occured. We\'ll fix it ASAP.');
    }
});