Centering thumbnails in NextGen Gallery

I’ve looked through previous posts on this topic, tried the suggestions and I just can’t get them to center. Below is what I have in my stylesheet now, can anyone help?

.ngg-galleryoverview {
text-align: center;
display:block;
}

.ngg-gallery-thumbnail-box {
display: inline-block;
}

.ngg-gallery-thumbnail {
margin-right: 5px;
text-align: center;
display: inline-block;
}