Paste the following css to your style.css
.ngg-gallery-thumbnail-box { margin: 5px .5%!important; width: 32.3%!important; }
.ngg-gallery-thumbnail a { display: block; overflow: hidden; }
.ngg-gallery-thumbnail a img { border: 1px solid #8eb83b!important; width: 95%!important; padding: 2%!important; margin: 0 auto!important; }
Or like this:
.ngg-galleryoverview { padding: 1px 0 0 1px !important; }
.ngg-gallery-thumbnail-box {
height: auto;
margin: 0px 0 0 0px !important;
max-width: 20%;
width: 20%;
padding: 0px !important;
background-color: #ffffff !important;
float: left;
}
.ngg-gallery-thumbnail-box:nth-child(5n+1) { clear: both; }
.ngg-gallery-thumbnail {
float: left;
margin-right: 3px !important;
margin-bottom: 3px !important;
text-align: center;
}
.ngg-gallery-thumbnail img {
border: 1px solid #ccc !important;
height: 100%;
margin: 0 !important;
max-width: 100%;
padding: 0px !important;
width: 100%;
}
.ngg-gallery-thumbnail a:hover img {
zoom: 1;
filter: alpha(opacity=70);
opacity: 0.7;
}
img.ngg-singlepic {
background-color: #FFFFFF !important;
border: 1px solid #ccc !important;
padding: 5px !important;
}