switch_theme.module

<?php
use Drupal\Core\Access\AccessResult;
use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\Core\Form\FormStateInterface;

function switch_theme_page_attachments(array &$attachments) {
$attachments['#attached']['library'][] = 'switch_theme/switch_theme';
}

function switch_theme_theme($existing, $type, $theme, $path){
   return [
    'switch_theme_block'     => [
      'variables' => [
        // define defaults for any variables you want in the twig file
        'attributes' => [
           'class' => ['switch_theme'],
         ], //etc
      ],
    ],
  ];

}

?>