Why doesn't my lightbox centre on the page?

You can see it here - random-sheep-sandbox.com/HWR/Gallery

Hello Daryl,
The issue is caused by your random-sheep-sandbox.com/HWR/css/style.css file. It resets everything:

html, body, div, span, applet, object, iframe, h1, h2, h3, h4, h5, h6, p, blockquote, pre, a, abbr, acronym, address, big, cite, code, del, dfn, em, img, ins, kbd, q, s, samp, small, strike, sub, sup, tt, var, dl, dt, dd, ol, ul, li, fieldset, form, label, legend, table, caption, tbody, tfoot, thead, tr, th, td, article, aside, canvas, details, embed, figure, figcaption, footer, header, hgroup, menu, nav, output, ruby, section, summary, time, mark, audio, video {
    border: 0 none;
    font: inherit;
    margin: 0;
    padding: 0;
    vertical-align: baseline;
}

The following properties send the lightbox to the top left:

    margin: 0;
    padding: 0;
    vertical-align: baseline;