Thank you for your answer!
The code works, but the result is that the gap for the rightmost thumbnails also increase.
But I’ve got the solution already. Since i also posted my question on the comment page on themeforest.net, your colleagues gave me the following code which does the job.
.av-large-gap.av-flex-size .av-masonry-entry .av-inner-masonry {
margin-right: 30px;
margin-bottom: 30px;
}
.av-masonry-container {
width: 103.5%;
}
Thank you for your help anyway :)
Max