Custom Install.php template
Here's the trick that Kickstart uses (from commerce_kickstart.install).
The actual styling is in the Shiny admin theme.

/**
 * Implements hook_install_tasks_alter().
 */
function commerce_kickstart_install_tasks_alter(&amp;$tasks, $install_state) {
  _commerce_kickstart_set_theme('shiny');
}

/**
 * Force-set a theme at any point during the execution of the request.
 *
 * Drupal doesn't give us the option to set the theme during the installation
 * process and forces enable the maintenance theme too early in the request
 * for us to modify it in a clean way.
 */
function _commerce_kickstart_set_theme($target_theme) {
  if ($GLOBALS['theme'] != $target_theme) {
    unset($GLOBALS['theme']);

    drupal_static_reset();
    $GLOBALS['conf']['maintenance_theme'] = $target_theme;
    _drupal_maintenance_theme();
  }
}