This is simple (wonder why authors hardcode body tag).
Open files albums.php, view_album.php and view_photo.php in text editor.
Find string that contain <body>.
Delete that string and write following code:
<?
includeHtmlWrap("body.tag");
?>
Then create file body.tag in html_wrap directory, containing your body tag.
Sure, you can change tag directly in PHP code, but i think this is very ugly and not re-usable.
This works only for "basic viewers part" of gallery, if you want changed body in all gallery - change other PHP files: slideshow.php, view_comments.php etc.
view_photo.php have some potential trouble source:
<? if ($fitToWindow) { ?>
<body onResize='doResize()'>
so, if you want this option - remove this code, and include doResize in your body tag (that written in body.tag file).
Sorry for my English, as always.