// Activate WordPress Maintenance Mode
function wp_maintenance_mode() {
    if (!current_user_can('edit_themes') || !is_user_logged_in()) {
        wp_die('<h1>Website Under Maintenance</h1><p>We are currently performing updates. Please check back soon.</p>', 'Maintenance Mode', array('response' => 503));
    }
}
add_action('get_header', 'wp_maintenance_mode');