_drupal_maintenance_theme

_drupal_maintenance_theme()

function _drupal_maintenance_theme() {
  // If the theme is already set, assume the others are set too, and do nothing.
  if (\Drupal::theme()->hasActiveTheme()) {
    return;
  }

  require_once __DIR__ . '/theme.inc';
  require_once __DIR__ . '/common.inc';
  require_once __DIR__ . '/unicode.inc';
  require_once __DIR__ . '/file.inc';
  require_once __DIR__ . '/module.inc';
  require_once __DIR__ . '/database.inc';
  Unicode::check();

  // Install and update pages are treated differently to prevent theming overrides.
  if (defined('MAINTENANCE_MODE') && (MAINTENANCE_MODE == 'install' || MAINTENANCE_MODE == 'update')) {
    if (drupal_installation_attempted()) {
      $custom_theme = $GLOBALS['install_state']['theme'];
    }
    else {
      $custom_theme = Settings::get('maintenance_theme', 'seven');
    }
  }
  else {
    // Use the maintenance theme if specified, otherwise attempt to use the
    // default site theme.
    try {
      $custom_theme = Settings::get('maintenance_theme', '');
      if (!$custom_theme) {
        $config = \Drupal::config('system.theme');
        $custom_theme = $config->get('default');
      }
    }
    catch (\Exception $e) {
      // Whatever went wrong (often a database connection problem), we are
      // about to fall back to a sensible theme so there is no need for special
      // handling.
    }
    if (!$custom_theme) {
      // We have been unable to identify the configured theme, so fall back to
      // a safe default. Bartik is reasonably user friendly and fairly generic.
      $custom_theme = 'bartik';
    }
  }

  $themes = \Drupal::service('theme_handler')->listInfo();

  // If no themes are installed yet, or if the requested custom theme is not
  // installed, retrieve all available themes.
  /** @var \Drupal\Core\Theme\ThemeInitialization $theme_init */
  $theme_init = \Drupal::service('theme.initialization');
  $theme_handler = \Drupal::service('theme_handler');
  if (empty($themes) || !isset($themes[$custom_theme])) {
    $themes = $theme_handler->rebuildThemeData();
    $theme_handler->addTheme($themes[$custom_theme]);
  }

  // \Drupal\Core\Extension\ThemeHandlerInterface::listInfo() triggers a
  // \Drupal\Core\Extension\ModuleHandler::alter() in maintenance mode, but we
  // can't let themes alter the .info.yml data until we know a theme's base
  // themes. So don't set active theme until after
  // \Drupal\Core\Extension\ThemeHandlerInterface::listInfo() builds its cache.
  $theme = $custom_theme;

  // Find all our ancestor themes and put them in an array.
  // @todo This is just a workaround. Find a better way how to handle themes
  //   on maintenance pages, see https://www.drupal.org/node/2322619.
  // This code is basically a duplicate of
  // \Drupal\Core\Theme\ThemeInitialization::getActiveThemeByName.
  $base_themes = [];
  $ancestor = $theme;
  while ($ancestor && isset($themes[$ancestor]->base_theme)) {
    $base_themes[] = $themes[$themes[$ancestor]->base_theme];
    $ancestor = $themes[$ancestor]->base_theme;
    if ($ancestor) {
      // Ensure that the base theme is added and installed.
      $theme_handler->addTheme($themes[$ancestor]);
    }
  }
  \Drupal::theme()->setActiveTheme($theme_init->getActiveTheme($themes[$custom_theme], $base_themes));
  // Prime the theme registry.
  Drupal::service('theme.registry');
}