Horizontal Center Aligning for WP Media Gallery

Hi! I’ve been trying to figure this out on my own with no resolve. ?? I’m fairly new so please be gentle! I’m trying to get the image gallery that i set up in wp to center horizontally.

https://www.northbrookmontessori.org/gallery-1/

It’s functioning perfectly with the exception that it’s floating left and I’ve tried a few dozen css tricks and nothing seems to be working. I’ve been scouring the forums trying to apply the suggestions to my page with no avail. Any help would be greatly appreciated! The modular theme has a custom CSS box that I have been using. Thanks!!!