Lightbox captions too long

I’m using captions for my images which are often quite long. The problem is that they are overlapping the “image x/y” and the navigation arrows. I need to adjust the maximum width of the caption, as well as the left and top margins, but I’m having trouble finding the right sections to edit in the stylesheet. I’ve tried everything that seemed relevant with no effect.

Open the first image to see what I mean:
https://www.aliastephen.com/lighting-design-portfolio/nine/

Thanks.