How to theme form in Drupal 8

https://www.bryanbraun.com/2013/08/17/using-hook-form-base-form-id-alter/

use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Render\Element;

/*
 * Implement hook_form_base_id_alter()
 */

function atadmin_form_node_form_alter(&$form, FormStateInterface $form_state) {
  $node = $form_state->getFormObject()->getEntity();
 if($form['#form_id'] == 'node_story_form' || $form['#form_id'] == 'node_story_edit_form'){
    $form['#theme'] = array('at_node_story_form');
 }elseif($form['#form_id'] == 'node_video_form' || $form['#form_id'] == 'node_video_edit_form'){
   $form['#theme'] = array('at_node_video_form');
 }elseif($form['#form_id'] == 'node_photo_gallery_form' || $form['#form_id'] == 'node_photo_gallery_edit_form'){
   $form['#theme'] = array('at_node_photo_gallery_form');
 }elseif($form['#form_id'] == 'node_poll_form' || $form['#form_id'] == 'node_poll_edit_form'){
   $form['#theme'] = array('at_node_poll_form');
 }
}

I implemented the hook "hook_form_base_id_alter" in my atadmin.theme file. Where atadmin is my theme name. Now I am going to theme the form of Story,video , photo_gallery content types , The Form Url would be , node/add/story,node/add/video etc

/**
 * Implements hook_theme()
 */
function atadmin_theme() {
    $themes['at_node_story_form'] = ['render element' => 'form'];
    $themes['at_node_video_form'] = ['render element' => 'form'];
    $themes['at_node_photo_gallery_form'] = ['render element' => 'form'];
    $themes['at_node_poll_form'] = ['render element' => 'form'];
   return $themes;
}

Implement hook_theme() , in the same file name Now create the twig files with the below names , Just replaces the theme key '_' to '-' like below ,

• at-node-story-form.html.twig
• at-node-video-form.html.twig
• at-node-photo-gallery-form.html.twig
• at-node-poll-form.html.twig

And The twig file code would be something like below, 

{{ form|without('field_related_stories','actions','advanced', 'footer','group_seo_meta_tags') }}

{{ form.group_section_category‎ }}  


    {{ form.group_custom_advanced }}
    {{ form.group_story_highlights }}
    {{ form.group_other_setting }}
    {{ form.group_story_date_time }}
    {{ form.group_story_social_media }}
    {{ form.group_story_other_setting }}
    {{ form.group_seo_meta_tags }}
    {{ form.field_related_stories }}{{ form.footer }}



    {{ form.actions }}