Insert the following code in the CSS area of the template. Specify the size of the thumbnail…medium, large, full size:
.gallery-columns-2.gallery-size-thumbnail, .gallery-columns-3.gallery-size-thumbnail, .gallery-columns-4.gallery-size-thumbnail {
display: table;
margin: 0 auto 20px;
text-align: center;
font-size: 0;
line-height: 0;
}