Changing Lightbox background to black in a Gallery Block

The lightbox default for images in any gallery block is a white translucent background. As I'm showing off artwork it works better with a solid black background. But I would like to have the same black background in a gallery block's lightbox function since it's much easier to manage rows of images. I have to resort to adding image blocks one by one which is much harder to arrange and make a neat clean row but allows for a lightbox with a black background.  

Can you share link to page where you inserted image block? I can take a look with code to change lightbox background