Bypass drupal theme with hook_menu() function?
Just to offer an alternative to Ayesh K's solution, you can set the delivery callback of your menu item


  The function to call to package the result of the page callback function and send it to the browser. Defaults to drupal_deliver_html_page() unless a value is inherited from a parent menu item. Note that this function is called even if the access checks fail, so any custom delivery callback function should take that into account. See drupal_deliver_html_page() for an example.