only your host/server admin will be able to tell you why it throws a 403
what i can tell you is this though

the wppizza.grid.css.php dynamically generates the css as per your settings and has a content type of

header(“Content-Type: text/css”);

it *might* be the case that due to the fact it’s a php file but with a text/css header your server throws the 403
it *might* also be the case that dues to the above your file permissions for this file have to be different

but again, only your server admin will be able to track down (by looking at log files etc) why exactly the issue exists