It is passing '/mdd/cc-none' into the w.php script in the $_GET['css'] field. If that script doesn't reference that field then it does nothing. If the script does reference the field then it does whatever the script is supposed to do with that field using that value.
If that field is used then the first step in the code should be validating that it contains a meaningful value for the script to use and so either '/mdd/cc-none' will pass validation because it is a valid value in which case the script will use it or it will fail validation and the script will produce an error.
The only way a hacker could gain any benefit by entering that would be if the script uses the field and whoever wrote the script neglected to validate it - since it can easily have any value supplied by anyone and so it is absolutely essential that it be validated FIRST before doing anything else with it.