Checks that current theme files 'index.php' and 'style.css' exists.
Does not check the 'default' theme. The 'default' theme should always exist or should have another theme renamed to that template name and directory path. Will switch theme to default if current theme does not validate. You can use the 'validate_current_theme' filter to return false to disable this functionality.
<?php validate_current_theme() ?>
validate_current_theme() is located in wp-includes/theme.php.