You have to set a fixed width for the gallery container that is equal to the combined width of how many thumbnails you want to display across.
Using easy math:
thumbnails are 100 x 100
margin is 10px on each making them 120px wide each
show six thumbnails across = width:720px
note these are not your dimensions but using firebug I was able to get yours looking pretty good using a width of 720px – it showed 5 across at that width.
Here is the CSS I used:
.ngg-galleryoverview {
overflow: hidden;
width: 720px;
clear: both;
display: block !important;
margin: 0 auto;
}
This CSS can be found in this file:
https://www.stayinwonderland.com/wp-content/plugins/nextgen-gallery/products/photocrati_nextgen/modules/nextgen_basic_gallery/static/thumbnails/nextgen_basic_thumbnails.css?ver=3.8.4
Since your theme CSS is above wp_head() it loads the nextgen css afterward so you should put it there instead of in your theme’s CSS where it will be overridden by nextgen styles. Or ideally, move your stylesheet so it is enqueued after nextgen’s so yours can override the plugin styles.