Project featured image in a lightbox

Hi there !

I made a projects page and when I click on the magnifying glass (on the thumbnail) the picture appear in a single page, with the adress “https://…/uploads/123.jpg
And I would like to display the picture in a lightbox. How can I do?
I saw in the theme demo that normally it appears in a lightbox so it is possible…

Your theme is awesome, thank you !