Don't hard code minelli as install & update theme

includes/theme_maintenance.inc

_drupal_maintenance_theme()

  // Install and update pages are treated differently to prevent theming overrides.
  if (defined('MAINTENANCE_MODE') && (MAINTENANCE_MODE == 'install' || MAINTENANCE_MODE == 'update')) {
    $theme = 'minnelli';
  }

theme_install_page()

  $theme_path = 'themes/garland';

  return theme_render_template('themes/garland/maintenance-page.tpl.php', $variables);

theme_update_page()

  $theme_path = 'themes/garland';

  return theme_render_template('themes/garland/maintenance-page.tpl.php', $variables);

after changing these values I was able to show install and update pages with different theme..