Custom Gallery Style

Hello,

i’ve add a lightbox effect to default wordpress gallery using this code:

/*----------------------
FUNZIONI PERSONALIZZATE
----------------------*/

// Remove Gallery Styling
add_filter( 'gallery_style', 'my_gallery_style', 99 );

function my_gallery_style() {
    return "
";
}

add_filter( 'use_default_gallery_style', '__return_false' );

/* ---------------------
AGGIUNGRE ATTRIBUTO LIGHTBOX
-----------------------*/

// Init Lightbox on Gallery Elements
add_action( 'wp_print_footer_scripts', 'add_lightbox_to_wp_gallery' );
function add_lightbox_to_wp_gallery() {

echo '
 <script type="text/javascript">
  /* <![CDATA[ */
  jQuery(document).ready(function($) {
   $(".gallery-icon a").attr("data-lightbox","my-gallery");
  });
 /* ]]> */
</script>';
}

And add to header.php the script of Lightbox js plugin. Now i like to custom the style of wordpress gallery to have less space between the colum (now i use 3 columns) and custom visual of gallery.

I like to have this effect https://www.dropbox.com/s/up3xipl1fdf3o6g/esempio.jpg?dl=0 how can i do?