Vanilla 2

I asked the developers of my paid WordPress theme for help in adjusting the size of prettyphoto lightbox, but they referred me here. Since prettyphoto is incorporated into their theme and I see no options to adjust prettyphoto, I'd like some guidance on how to enlarge the size of lightbox when viewed on mobile. Just look at the following page for example:

http://prolineproductsinc.com/pro-line-building-products/
(browse down to photo gallery and click a thumnail)

On mobile, the lightbox is only 50% width of screen, which is too small. I know there's probably an easy way to change this, maybe in the .js file or .css file, but I don't know where. Any help would be appreciated!