#gallery-1 {
margin: auto;
}
#gallery-1 .gallery-item {
float: left;
margin-top: 10px;
text-align: center;
width: 50%;
}
#gallery-1 img {
border: 2px solid #cfcfcf;
}
#gallery-1 .gallery-caption {
margin-left: 0;
}
/* see gallery_shortcode() in wp-includes/media.php */
...