who does that?

Drupal hardcodes minelli to be the theme for the update.php and install.php pages. Stupid.

• Minnelli is always used for the initial install and update operations. In
• other cases, "settings.php" must have a "maintenance_theme" key set for the

• $conf variable in order to change the 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';
  }