/**
* Implementation of HOOK_theme().
*/
function THEMENAME_theme() {
return array(
'comment_form' => array(                       
      'arguments' => array('form' => NULL),
    ),
  );
}
 
// theme the comment form
function THEMENAME_comment_form($form) {
  // Make the text-area smaller.
  $form['comment_filter']['comment']['#rows']   = 5;
  // Change the text-area title
  $form['comment_filter']['comment']['#title']  = t('Your message');
  // Remove input formats information.
  $form['comment_filter']['format'] = NULL;
  // Removes homepage field
  unset($form['homepage']);
  // Removes preview button
  unset($form['preview']);             
  return drupal_render($form);
}