Why is my theme function not called (not in template file)?
arguments was for Drupal 6, you need to use variables for Drupal 7.  One of variables or render element is required, hence nothing is showing up at the moment as the item isn't making it into the theme registry.

function activit_theme() {

  return array(

      'activit_package_teaser' =&gt; array(
          'variables' =&gt; array('node' =&gt; NULL),
          'type' =&gt; 'module'
      ),

  );
}