Drupal 6 make a content edit form be in a specified theme
Implement hook_form_alter in custom module and check for current theme :

global $theme;
function mymodule_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id == 'example_node_form_id' &amp;&amp; $theme != 'theme_machine_name') {
    // redirect anywhere using drupal_goto.
  }
}