Create inset border around Hero Image on mobile

Site URL: https://raccoon-tarantula-6j7y.squarespace.com/

Hi, I want to create a border around my slideshow gallery on mobile.

Instead of it being full width, I would like to add an inset spacing, like the red square I draw on the attachement.

pass: giuseppe

Add to Design > Custom CSS

body.homepage .gallery-fullscreen-slideshow {
    border: 10px solid red;
}