Why is overlay caption text colour black?

Site URL: https://grasshopper-saffron-ffl9.squarespace.com/

When I choose " heading 4" for my text size in the overlay caption, it is black. I cant find a place to change it. None of the other sizes are doing this. I have no custom CSS anywhere so Im not sure whats happened. 

Thanks for your help!

Hi. Can you share link to page in screenshot? We can check easier