Add borders to individual images on Gallery Grid: Simple

Site URL: https://www.hanroco.com/art-direction

i added a 2px black border to all of the images on my site (https://www.hanroco.com). i'm trying to do the same for the images on this page (https://www.hanroco.com/art-direction) which are galleries rather than inline images. the best i can get is a border surrounding each entire gallery using .gallery-grid-wrapper {border: 2px solid #000}. any help would be very much appreciated!

Add to Design > Custom CSS

/* Art direction image border */
body#collection-5ff66cfe393fd96834e306ed #page img {
    border: 10px solid green;
}