@Joseph: Sorry we have no control on this. This is default gallery from WordPress.
You can add the following css to make it center:
.attachment .entry-attachment {
text-align: center;
}
.attachment .entry-attachment a {
display: inline-block;
margin: 0 auto;
}
That number below the gallery is the Caption in theme image. If you don’t want that then edit that image and remove the caption.
To remove date from attachment, you can add the following css in “Appearance => Theme Options => Custom CSS” box:
.attachment .entry-meta { display: none; }
But to remove the link to full image, you cannot do that from Custom CSS. For this you need to build child theme. Check this http://catchthemes.com/blog/create-child-theme-wordpress/ for child theme. The copy image.php in yoru child theme and edit that link.
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