The lightbox close button should have an opposite border color.
At the moment, the button has only one color. So if you choose white and the picture is also white, you don’t see him.
The back and forward buttons do have an opposite border color, so you can always see them.
Example, where you can see the close button.
Another example, where you can’t see the close button, but can see the back and forward arrows.
You can test it on my website. For sure, you have to adjust the browser window size to see the different states. You can easily test it with the last picture in the first picture row.