Switching Grid to RTL (right-to-left) for Arabic-based website

Great theme!! Many thanks!!!

I am trying to use this great theme with an Arabic-based, RTL website: http://www.mar7ab10.com/

So far so good and with minimal CSS work it looks fantastic.
However, I would be very happy to set the masonry grid to RTL as well, such that the most recent post shows up at the top right corner instead of top left corner.

I am not an expert on JS and masonry but, on a different site, I made the following change that worked flawlessly:
Change from:

 $container.masonry({
   itemSelector : '.grid-box',
 });

To:

 $container.masonry({
   itemSelector : '.grid-box',
   isOriginLeft: false
});

But in Pinboard I couldn’t find this script.
Can you please help?

Thanks!