function yourthemename_textarea($variables) {
  $element = $variables['element'];
  $element['#attributes']['name'] = $element['#name'];
  $element['#attributes']['id'] = $element['#id'];
  $element['#attributes']['cols'] = $element['#cols'];
  $element['#attributes']['rows'] = $element['#rows'];
  _form_set_class($element, array('form-textarea'));

  $wrapper_attributes = array(
    'class' => array('form-textarea-wrapper'),
  );

  // Add resizable behavior.
  if (!empty($element['#resizable'])) {
    $wrapper_attributes['class'][] = 'resizable';
  }

  $output = '<div' . drupal_attributes($wrapper_attributes) . '>';
  $output .= '<textarea' . drupal_attributes($element['#attributes']) . '>' . check_plain($element['#value']) . '</textarea>';
  $output .= '</div>';
  return $output;
}