if your looking to just make the detailed images box have a black background do this for v3.5.x
Code:
{* $Id: product_images.tpl,v 1.10 2003/08/11 10:45:10 svowl Exp $ *}
{if $images ne ""}
{capture name=dialog}
{* added this - start *}
<table width="100%" border="0" align="center" cellpadding="0" cellspacing="0" bgcolor="#000000">
<tr>
<td>
{* added this - end *}
<div align=center>
{section name=image loop=$images}
{if $images[image].avail eq "Y"}
{if $images[image].tmbn_url}
[img]{$images[image].tmbn_url}[/img]
{else}
[img]{$xcart_web_dir}/product_image.php?imageid={$images[image].imageid}[/img]
{/if}
{/if}
{/section}
</div>
{* added this - start *}
</td>
</tr>
</table>
{* added this - end *}
{/capture}
{if $smarty.capture.dialog ne ""}
{include file="dialog.tpl" title=$lng.lbl_detailed_images content=$smarty.capture.dialog extra="width=100%"}
{/if}
{/if}
all middle sections have the dialog.tpl file wrapped around them so adjusting that file will adjust all of them