How can I add class for div .form-item?
You can override the output of theme_form_element() function in your theme's template.php file. Just copy the function from includes/form.inc file and put it in template.php as MYTHEME_form_element(), and you can now change classes for only text fields with something like this before initializing $output variable :

if($element['#type'] == 'textfield'){
  // Add a class specific to textfields
  $attributes['class'][] = 'custom-textfield-class';
}


This code is not tested, but it should work. You should clear drupal cache after implementing the function in template.php.