print theme_get_setting in php-as-css file
Below code worked for me ! Make sure there is a value for setting "theme_custom_background_path" in your theme settings. For example i can able to get logo path of theme using below code


Created a file called get_setting.php inside my Drupal Root
Copy pasted below code
Executed in browser.


Code

define('DRUPAL_ROOT', getcwd());
include_once DRUPAL_ROOT . '/includes/bootstrap.inc';
drupal_bootstrap(DRUPAL_BOOTSTRAP_FULL);
$theme_settings = theme_get_setting('logo');
print "body { background: url(" . $theme_settings . ");}";