Do you have a link to this page?
There’s a “coming soon” plug in. But I’ll “open” the website for a short time. Please, check here: http://www.kvaskovani.cz/test-gallery/
Normally I would say don’t edit the theme file directly, but this will be part of the next update. Open the theme’s function.php file and look for this:
.img-intro img,.img-intro-left img,.img-intro-right img,.img-intro-none img,.img-full-left img,
.img-full-right img,.img-full-none img,
.imageborder,.contact-image img,.entry-attachment img,img.alignnone,img.alignright,
img.alignleft,img.aligncenter,
div.wp-caption img {border-color:<?php echo get_theme_mod( 'imgborder', '#78a5b6' ); ?>;}
Right after .wp-caption img add a comma then this:
.gallery .gallery-icon img.attachment-thumbnail
Thank you very much André. That worked like a charm !