The lightbox is supported by default on blog pages and portfolio pages like this one on the preview site –
https://www.livemeshthemes.com/extinct/portfolio-3-columns-sortable/
When you hover over the images, there should be a plus sign which shows up along with the title which when clicked, opens up the lightbox image.
Single portfolio pages do not support lightbox by default although there is a way to enable it by specifying the right markup in the content.