Trying to add caption overly to gallery

I'm new to Squarespace and trying to help a friend with their site.  I'm trying to add caption overlays to the image gallery.  I've tried adding css code, but it seems like all the videos/forums I see the interface of Squarespace looks different.  I add the caption to the image, but when I add the CSS code nothing changes, the caption just shows below the image.

Can you share link to page where you use image gallery? We can check problem easier.