Image container color

Please scroll down, the 3 images in a row is a gallery. Click an image, the lightbox opens. The container background color for the image is white, how to make it black?

I have the same plug in on another site, the background is black. https://www.alexastudios.net/portraits/ again, scroll down for the thumbnails.

I did check all the settings inside the gallery options but found none relating to color.

Thank you!

The page I need help with: [log in to see the link]