Implements hook_form_FORM_ID_alter().
Alters the theme form to use the admin theme on node editing.
node_form_system_themes_admin_form_submit()
function node_form_system_themes_admin_form_alter(&$form, &$form_state, $form_id) {
$form['admin_theme']['node_admin_theme'] = array(
'#type' => 'checkbox',
'#title' => t('Use the administration theme when editing or creating content'),
'#default_value' => variable_get('node_admin_theme', '0'),
);
$form['#submit'][] = 'node_form_system_themes_admin_form_submit';
}