Theme process function for theme_maintenance_field().
The variables array generated here is a mirror of template_process_html(). This processor will run its course when theme_maintenance_page() is invoked.
function template_process_maintenance_page(&$variables) {
$variables['head'] = drupal_get_html_head();
// While this code is used in the installer, the language module may not be
// enabled yet (even maybe no database set up yet), but an RTL language
// selected should result in RTL stylesheets loaded properly already.
$variables['css'] = $css = drupal_add_css();
include_once DRUPAL_ROOT . '/core/modules/language/language.module';
language_css_alter($css);
$variables['styles'] = drupal_get_css($css);
$variables['scripts'] = drupal_get_js();
}