Magnific Popup – Add Caption to Images

My theme uses Magnific Popup as follows in theme.js

  /* add lightbox for blog galleries */
  $(".gallery a[href$='.jpg'],.gallery a[href$='.jpeg'],.featured-item a[href$='.jpeg'],.featured-item a[href$='.gif'],.featured-item a[href$='.jpg'], .page-featured-item .slider a[href$='.jpg'], .page-featured-item a[href$='.jpg'],.page-featured-item .slider a[href$='.jpeg'], .page-featured-item a[href$='.jpeg'], .gallery a[href$='.png'], .gallery a[href$='.jpeg'], .gallery a[href$='.gif']").parent().magnificPopup({
    delegate: 'a',
    type: 'image',
    tLoading: '<div class="loading dark"><i></i><i></i><i></i><i></i></div>',
    mainClass: 'my-mfp-zoom-in',
    gallery: {
      enabled: true,
      navigateByImgClick: true,
      preload: [0,1] // Will preload 0 - before current, and 1 after the current image
    },
    image: {
      tError: '<a href="%url%">The image #%curr%</a> could not be loaded.',
      verticalFit: false
    }
  }); 

I would like to modify this to add the caption of each image in the WordPress Gallery. I know I should be using the selector.

image: {
            titleSrc: 
}

Also I don’t want to edit this code directly as it’s part of a theme. Could I use a function or perhaps the footer to append my changes?