How to theme back end forms created by a custom module? I want to be able to theme a custom form I create using a custom module that builds a form. The form is created programmatically using the code below:

public function buildForm(array $form, FormStateInterface $form_state) {

    $form['name'] = array(
      '#type' =&gt; 'textfield',
      '#title' =&gt; $this-&gt;t('Name'),
      '#required' =&gt; TRUE
    );

    $form['address'] = array(
      '#type' =&gt; 'textfield',
      '#title' =&gt; $this-&gt;t('Address'),
      '#required' =&gt; TRUE
    );

    $form['actions']['submit'] = array(
      '#type' =&gt; 'submit',
      '#value' =&gt; $this-&gt;t('Submit'),
    );

    return $form;
}


This form is rendered on the Drupal 8 back-end and uses the admin theme classy BEGIN OUTPUT from 'core/themes/classy/templates/form/form.html.twig'. However there isn't a override file name suggestion. 

How would I theme this form?
One option is to add a prefix and suffix to your form arrays

$form['name'] = array(
  '#type' =&gt; 'textfield',
  '#title' =&gt; $this-&gt;t('Name'),
  '#required' =&gt; TRUE
  '#prefix' =&gt; '&lt;div class="something"&gt;',     
  '#suffix' =&gt; '&lt;/div&gt;',
);

$form['#attached']['css'] = array(
 'path/to/my.css' =&gt; array(
   'group' =&gt; CSS_AGGREGATE_THEME,
   'weight' =&gt; 999
 ),
);


my.css

.something { 
  border: solid red;
}