Hi Mike,
thank you for the quick answer. Maybe I’ve found another solution with the help of this forum:

I added this to functions.php

function add_class_to_foogallerylightbox() {
?>
<script type=”text/javascript”>
jQuery(document).ready(function() {
jQuery(‘div.foogallery’).addClass(‘noLightbox’);
});
</script>
<?php
}
add_action(‘wp_footer’, ‘add_class_to_foogallerylightbox’);

Seems to work – what do you think?