How theming off-line page? I read API and put files theme folder.

maintenance-page.tpl.php works well,

but maintenance-page--offline.tpl.php don't work.

How theming off-line page ?
Are you sure that your database server was offline when you tested ? The API description says 


  "maintenance-page--offline.tpl.php" can be used when the database is offline to hide errors and completely replace the content.