How to customize the comments form? For example, How can I add some classes or hide labels and instead of it add placeholder with using of ThemeName_preprocess_comment to 'subject' and 'name' comments form elements?

I have tried to usedrupal_set_message(kint($variables)); in ThemeName_preprocess_comment section, but it cause the empty screen erroe in my site.

So how can I add some classes to default (or my own created) comments elements so I can target them for adding CSS style.

How can I target default comments form element in CSS?
if you use Drupal7, you should not use 

drupal_set_message(kint($variables));


but

kint($variables) 


or 

dpm($variables)


An example that you can see, enter in the following link:
 https://api.drupal.org/api/drupal/modules%21comment%21comment.module/function/template_preprocess_comment/7.x

function bartik_preprocess_comment(&amp;$variables) {

  $variables['classes_array'][] = 'myclass';
  dpm($variables);

}


You can also create a file "comment-wrapper.tpl.php" and add the classes you want and placeholder.

Example:

&lt;?php

/**
 * @file
 * Bartik's theme implementation to provide an HTML container for comments.
 *
 * Available variables:
 * - $content: The array of content-related elements for the node. Use
 *   render($content) to print them all, or
 *   print a subset such as render($content['comment_form']).
 * - $classes: String of classes that can be used to style contextually through
 *   CSS. It can be manipulated through the variable $classes_array from
 *   preprocess functions. The default value has the following:
 *   - comment-wrapper: The current template type, i.e., "theming hook".
 * - $title_prefix (array): An array containing additional output populated by
 *   modules, intended to be displayed in front of the main title tag that
 *   appears in the template.
 * - $title_suffix (array): An array containing additional output populated by
 *   modules, intended to be displayed after the main title tag that appears in
 *   the template.
 *
 * The following variables are provided for contextual information.
 * - $node: Node object the comments are attached to.
 * The constants below the variables show the possible values and should be
 * used for comparison.
 * - $display_mode
 *   - COMMENT_MODE_FLAT
 *   - COMMENT_MODE_THREADED
 *
 * Other variables:
 * - $classes_array: Array of html class attribute values. It is flattened
 *   into a string within the variable $classes.
 *
 * @see template_preprocess_comment_wrapper()
 */
?&gt;
&lt;?php 
    //add placeholder
    $content['comment_form']['subject']['#attributes']['placeholder'] = t('Title');
    $content['comment_form']['comment_body']['und'][0]['value']['#attributes']['placeholder'] = t('Comment Body');
  ?&gt;
&lt;div id="comments" class="&lt;?php print $classes; ?&gt;"&lt;?php print $attributes; ?&gt;&gt;
  &lt;?php if ($content['comments'] &amp;&amp; $node-&gt;type != 'forum'): ?&gt;
    &lt;?php print render($title_prefix); ?&gt;
    &lt;h2 class="title"&gt;&lt;?php print t('Comments'); ?&gt;&lt;/h2&gt;
    &lt;?php print render($title_suffix); ?&gt;
  &lt;?php endif; ?&gt;

  &lt;?php print render($content['comments']); ?&gt;

  &lt;?php if ($content['comment_form']): ?&gt;
    &lt;h2 class="title comment-form "&gt;&lt;?php print t('Add new comment'); ?&gt;&lt;/h2&gt;
    &lt;?php print render($content['comment_form']); ?&gt;
  &lt;?php endif; ?&gt;


&lt;/div&gt;