gallery arrow background size

I would like the background for simple gallery arrows to extend the height of photos (instead of the little square box behind arrows)  so kind of a rectangle.  This is for a gallery section not block.  I want to keep the background the same as the default (white-medium opacity).  Can't find any CSS for this in a search.  any help would be greatly appreciated.

 Add to Website > Website Tools > Custom CSS

.gallery-lightbox-background {
    opacity: 1 !important;
}

thank you so much!