Its a cell padding or spacing issue. I've looked at stylevar and its set to 6px but something in the css is overriding it from the custom header.. i'm working on it now.
This code is somehow overriding the stylevar setting in the forum, i have no idea how yet.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>Overclocking Wiki, The Independent Overclocking Edge - Home</title>
<meta name="description" content="Overclocking Wiki, nvidia, ati, vista, microsoft, forum, synaps3, logo, graphics designer, .gif, jpg, jpeg, photoshop" />
<meta name="keywords" content="Overclocking Wiki, nvidia, ati, vista, microsoft, forum, synaps3, logo, graphics designer, .gif, jpg, jpeg, photoshop" />
<meta name="Generator" content="Joomla! - Copyright (C) 2005 - 2006 Open Source Matters. All rights reserved." />
<meta name="robots" content="index, follow" />
<base href="http://www.overclockingwiki.org/forums/" />
<script src="http://www.overclockingwiki.org/joomla/includes/js/joomla.javascript.js" type="text/javascript"></script>
<link rel="alternate" type="application/rss+xml" title="Overclocking Wiki, The Independent Overclocking Edge" href="http://www.overclockingwiki.org/joomla/index2.php?option=com_rss&feed=RSS2.0&no_html=1" />
<link rel="shortcut icon" href="http://www.overclockingwiki.org/joomla/images/ocw.ico" />
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<meta http-equiv="Content-Style-Type" content="text/css" />
<link href="http://www.overclockingwiki.org/joomla/templates/ja_xenia/css/template_css.css" rel="stylesheet" type="text/css" />
<link href="http://www.overclockingwiki.org/joomla/templates/ja_xenia/css/colors/blue.css" rel="stylesheet" type="text/css" />
<link href="http://www.overclockingwiki.org/joomla/templates/ja_xenia/css/ja-news.css" rel="stylesheet" type="text/css" />
<script language="javascript" type="text/javascript" src="http://www.overclockingwiki.org/joomla/templates/ja_xenia/scripts/ja.script.js"></script>
<script type="text/javascript">
var currentFontSize = 3;
</script>
<link href="http://www.overclockingwiki.org/joomla/templates/ja_xenia/ja_splitmenu/ja-splitmenu.css" rel="stylesheet" type="text/css" />
<!--[if lte IE 6]>
<style type="text/css">
.clearfix { height: 1%;}
</style>
<![endif]-->
<!--[if gte IE 7.0]>
<style type="text/css">
.clearfix { display: inline-block;}
</style>
<![endif]-->
</head>
<body id="bd" class="auto fs3">
<a title="Skip to content" href="http://www.overclockingwiki.org/joomla/#Content" style="display: none;">Skip to content</a>
<a name="Top" id="Top"></a>
<div id="ja-wrapper">
<div id="ja-wrapper-inner">
<!-- BEGIN: HEADER -->
<div id="ja-headerwrap">
<div id="ja-header" class="clearfix">
<h1>
<a href="index.php">
<img src="http://www.overclockingwiki.org/joomla/templates/ja_xenia/images/logo-blue.jpg" alt="Overclocking Wiki, The Independent Overclocking Edge" />
</a>
</h1>
<div id="ja-topnavwrap" class="clearfix">
</div>
</div>
</div>
<!-- END: HEADER -->
<!-- BEGIN: MAIN NAVIGATION -->
<div id="ja-mainnavwrap" class="clearfix">
<div id="ja-mainnav">
<ul id="ja-splitmenu" class="mainlevel"><li class="active" id="menu1"><a href="/joomla/" title="Home"><span>Home</span></a></li>
<li id="menu2"><a href="http://www.overclockingwiki.org/joomla/blogs/" title="Blogs"><span>Blogs</span></a></li>
<li id="menu16"><a href="http://www.overclockingwiki.org/joomla/external/forums.html" title="Forums"><span>Forums</span></a></li>
<li id="menu7"><a href="index.php?option=com_content&task=new§ionid=1&Itemid=7" title="Submit"><span>Submit</span></a></li>
<li id="menu18"><a href="http://www.overclockingwiki.org/joomla/component/option,com_docman/Itemid,18/" title="Files"><span>Files</span></a></li>
</ul></div>
<div id="ja-usertoolswrap">
<span class="ja-sitetool">Site Tools</span>
<div id="ja-usertools" style="left:-60px">
<span class="ja-usertools"> <a href="#Narrow" onclick="return false;"><img title="Narrow screen" src="http://www.overclockingwiki.org/joomla/templates/ja_xenia/images/blue/user-screen1.gif" alt="Narrow screen resolution" id="ja-tool-narrow" onclick="changeToolHilite(curtool, this);curtool=this;setScreenType('narrow');" /></a>
<a href="#Wide" onclick="return false;"><img title="Wide screen" src="http://www.overclockingwiki.org/joomla/templates/ja_xenia/images/blue/user-screen2.gif" alt="Wide screen resolution" id="ja-tool-wide" onclick="changeToolHilite(curtool, this);curtool=this;setScreenType('wide');" /></a>
<a href="#Auto" onclick="return false;"><img title="Auto adjust screen size" src="http://www.overclockingwiki.org/joomla/templates/ja_xenia/images/blue/user-screen3-hilite.gif" alt="Auto adjust screen size" id="ja-tool-auto" onclick="changeToolHilite(curtool, this);curtool=this;setScreenType('auto');" /></a>
<a href="#Increase" onclick="return false;"><img title="Increase font size" src="http://www.overclockingwiki.org/joomla/templates/ja_xenia/images/blue/user-increase.gif" alt="Increase font size" id="ja-tool-increase" onclick="changeFontSize(1); return false;" /></a>
<a href="#Decrease" onclick="return false;"><img title="Decrease font size" src="http://www.overclockingwiki.org/joomla/templates/ja_xenia/images/blue/user-decrease.gif" alt="Decrease font size" id="ja-tool-decrease" onclick="changeFontSize(-1); return false;" /></a>
<a href="#Default" onclick="return false;"><img title="Default size" src="http://www.overclockingwiki.org/joomla/templates/ja_xenia/images/blue/user-reset.gif" alt="Default font size" id="ja-tool-reset" onclick="revertStyles(3); return false;" /></a>
</span>
<script type="text/javascript">
var curtool = document.getElementById('ja-tool-auto');
var curcolor = document.getElementById('ja-tool-bluecolor');
</script>
</div>
</div>
</div>
<div class="ja-mainnavshadow"></div>
<div id="ja-containerwrap">
<div id="ja-container" class="clearfix">
<!-- logo -->
<a name="top"></a>
<table border="10" width="$stylevar[outertablewidth]" cellpadding="10" cellspacing="10" align="center">
<tr>
<td align="$stylevar[left]"><a href="$vboptions[forumhome].php$session[sessionurl_q]"><img src="$stylevar[titleimage]" border="0" alt="$vboptions[bbtitle]" /></a></td>
<td align="$stylevar[right]">
</td>
</tr>
</table>
<!-- /logo -->
<!-- content table -->
$spacer_open
$_phpinclude_output
Edit: The problem is the the template_css.css file from the Joomla template was being used as the stylesheet. Editing that file with a 'padding: 10;' instead of a 'padding: 0;' seems to have solved the problem without causing any other problems. At least, not that I've found yet.
www.overclockingwiki.org/forums (http://www.overclockingwiki.org/forums) and www.overclockingwiki.org/joomla (http://www.overclockingwiki.org/joomla) for those who'd like to see.
Keep in mind, pasting that code in like I did means when I change the main menu in Joomla I'm not changing in the forums. So, even though Joomla is generating that menu it can be out of date.
This will have to be updated whenever I make a change to the menu in Joomla.