As of version 2.0.0 for the premium Definite theme, we have removed the built-in gallery lightbox. We found the third-party ColorBox lightbox was using the image “title” for captions and not the actual image caption.
If you would like a lightbox for your website and the Definite theme, we recommend checking out the WP Featherlight plugin. Currently, it works for the older classic WP Gallery shortcode and also the Block Images/Galleries. We are using it on the live demo for this theme and for the Rough Pixels website; for our tutorial screenshots.