Drupal 8 switch active theme programmatically

drupal8-change-active-theme-programmatically.php

<?php
/**
 * See original implementation http://cgit.drupalcode.org/mailsystem/tree/src/MailsystemManager.php#n60
 */
// Switch the theme to the configured mail theme.
$theme_manager = \Drupal::service('theme.manager');
$mail_theme = '[your theme name]';
$current_active_theme = $theme_manager->getActiveTheme();
if ($mail_theme && $mail_theme != $current_active_theme->getName()) {
  $theme_initialization = \Drupal::service('theme.initialization');
  $theme_manager->setActiveTheme($theme_initialization->initTheme($mail_theme));
}

try {
  // Do your actions here.
  // .....
}
finally {
  // Revert the active theme, this is done inside a finally block so it is
  // executed even if an exception is thrown during sending a mail.
  if ($mail_theme != $current_active_theme->getName()) {
    $theme_manager->setActiveTheme($current_active_theme);
  }
}

Thanks for sharing.
Will give a try to this for my pages where an alternative theme is needed.