Accessed in Appearance → Customize → General settings → Lightbox settings.
Customization option for default theme lightbox(lightGallery) are only available in Rife Pro.
Here you can set many things about appearance of lightbox used in theme – controls, icons, colors.
If you wish to use some other plugin/script for images and items, you can switch default lightbox off.
Keep in mind that your custom lightbox will require custom coding form you to make it work with theme same way as default lightbox.