doGetBaseThemes example

  protected function fillInSubThemeData(array &$themes, array $sub_themes) {
    foreach ($sub_themes as $name) {
      $sub_theme = $themes[$name];
      // The $base_themes property is optional; only set for sub themes.       // @see ThemeHandlerInterface::listInfo()       $sub_theme->base_themes = $this->doGetBaseThemes($themes, $name);
      // empty() cannot be used here, since static::doGetBaseThemes() adds       // the key of a base theme with a value of NULL in case it is not found,       // in order to prevent needless iterations.       if (!current($sub_theme->base_themes)) {
        continue;
      }
      // Determine the root base theme.       $root_key = key($sub_theme->base_themes);
      // Build the list of sub-themes for each of the theme's base themes.       foreach (array_keys($sub_theme->base_themes) as $base_theme) {
        $themes[$base_theme]->sub_themes[$name] = $sub_theme->info['name'];
      }