I don't know if this can me done but, I would love seeing the photos on my portfolio with a dark background everytime I click to visualize it, and I mean when you open a photo, not on the editor (you can there).
Now when you are in your porfolio to see how a picture looks, the background of the thumbnails is dark. As soon as you click to zoom the photo, the background is white. It is a huge difference having a dark one and a white one.
OK so you want the Lightbox image viewer to have a dark background instead of a light background. Open your Portfolio editing portal. You will find Lightbox under Settings > Site Options. Adjust color & opacity as desired. See link below for details.