Theming of Drupal 6 forms is for web designer nearly the same as breathing. The way of theming form is not much complicated - in custom module is needed just form_alter function. The important part is - where in $form array is variable webdesigner need to change and how is its name? Just use this simple function...

<?php
  function mymodule_form_alter(&$form, $form_state, $form_id) {
    switch ($form_id) {
      case 'name_of_form':
        print '<pre>';
        print_r($form);
        print '</pre>';
        break;
    }
  }
?>