Allows you to define a theme_callback and change default theme. Useful when using Admin theme for node/edit, but you want forums to use default.

menu_get_item_alter-custom-theme.php

<?php

/**
 * Implements hook_menu_get_item_alter().
 */
function mymodule_menu_get_item_alter(&$router_item, $path, $original_map) {
  if ($router_item['path'] == 'node/add/forum') {
    $router_item['theme_callback'] = 'mymodule_menu_theme_callback';
  }
}

function mymodule_menu_theme_callback() {
  return 'default_theme';
}