Copy the default template file from modules/system/maintenance-page.tpl.php to your theme template folder sites/all/themes/YourThemeName/templates/maintenance-page.tpl.php.Change it the way you want. The text you entered in the Administration UI admin/config/development/maintenance will be available as<?php print $content; ?>in this template file.Add the following configuration$conf['maintenance_theme'] = 'YourThemeName';to your sites/default/settings.php file.Then clear the cachedrush cc allThat's it!You can also modify the offline screen when the database connection fails by creating the file sites/all/themes/YourThemeName/templates/maintenance-page--offline.tpl.phpMore information: http://api.drupal.org/api/drupal/includes!theme.inc/function/template_preprocess_maintenance_page/7
