How to make my new administration page use my administration theme?
There's a permission named View the administration theme. This should be given to the accountant role to allow users with that role to view the admin theme instead of the site theme.

If that permission is granted, then check the URL for the page you've created. A last resort would be to check out the documentation for hook_admin_paths(). This hook allows you to specify paths that should use the admin theme and/or use the Overlay (if you've got it enabled). Something like the following would work:

/**  
 * Implements hook_admin_paths().  
 */ 
function MY_MODULE_admin_paths() {   

  $admin_paths = [
    'my/new/admin/view' =&gt; TRUE,
  ];

  return $admin_paths; 

}