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// Temp Maintenance - with http response 503 (Service Temporarily Unavailable)
// This will only block users who are NOT an administrator from viewing the website.
function wp_maintenance_mode()
{
  if(!current_user_can('edit_themes') || !is_user_logged_in())
  {
    wp_die('Maintenance, please come back soon.', 'Maintenance - please come back soon.', array('response' => '503'));
  }
}

add_action('get_header', 'wp_maintenance_mode');