drupal_maintenance_theme()
drupal/includes/bootstrap.inc, line 845
Enables use of the theme system without requiring database access. Since there is not database access no theme will be enabled and the default themeable functions will be called. Some themeable functions can not be used without the full Drupal API loaded. For example, theme_page() is unavailable and theme_maintenance_page() must be used in its place.
function drupal_maintenance_theme() {
global $theme;
require_once './includes/path.inc';
require_once './includes/theme.inc';
require_once './includes/common.inc';
require_once './includes/unicode.inc';
unicode_check();
$theme = '';
}