How to use theming in other module
You should use the drupal_get_form() to render the node form.
For art content type.

$items['node/add/add-art'] = array(
  'title' =&gt; 'Add art work',
  'parent' =&gt; 'admin/commerce/manage-products/actions',
  'type' =&gt; MENU_LOCAL_TASK | MENU_NORMAL_ITEM,
  'page callback' =&gt; 'drupal_get_form',
  'page arguments' =&gt; array('art_node_form'),
  'access callback' =&gt; TRUE,
  'file' =&gt; drupal_get_path('module', 'node') . '/node.pages.inc'
);