You can theme the "Site-Offline" page in your Drupal site by modifying the maintenance-page.tpl.php file in your theme. But you also need to enable that maintenance-page.tpl.php file, because drupal will use the Minelli maintenance file by default.
To enable it open settings.php and uncomment this line
# 'maintenance_theme' => 'minnelli',
then change it to this
$conf['maintenance_theme'] = 'themename';
where "themename" is your theme's name.