Fais une copie de ton template Header avant de copier le code que je te fourni plus bas ;)
Remplaces ensuite le template Header par celui là
<link href="http://www.soltel.be/templates/ja_sargas/css/template_css.css" rel="stylesheet" type="text/css" />
<script language="javascript" type="text/javascript" src="http://www.soltel.be/templates/ja_sargas/scripts/ja.script.js"></script>
<link href="http://www.soltel.be/templates/ja_sargas/css/colors/default.css" rel="stylesheet" type="text/css" />
<script type="text/javascript">
var currentFontSize = 3;
</script>
<link href="http://www.soltel.be/templates/ja_sargas/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]-->
<script type="text/javascript" src="http://www.soltel.be/templates/ja_sargas/scripts/opacity.js"></script>
</head>
<body id="bd" class="wide fs3">
<a title="Skip to content" href="http://www.soltel.be/#Content" style="display: none;">Skip to content</a>
<!-- logo -->
<a name="top" id="Top">></a>
<div id="ja-headerwrap">
<div id="ja-header" class="clearfix">
<h1>
<a href="index.php">
<script type="text/javascript">
od_displayImage('sitelogo', 'http://www.soltel.be/templates/ja_sargas/images/logo', 198, 94, '', 'Soltel, portail sur la télécommunication en Belgique.');
</script>
</a>
</h1>
<div id="ja-sh">
<div id="ja-usertoolswrap">
<div id="ja-usertools">
<span class="ja-usertools">
<a href="#Narrow" onclick="setScreenType('narrow');return false;"><img title="Narrow screen" src="http://www.soltel.be/templates/ja_sargas/images/default/user-screen1.gif" alt="Narrow screen resolution" id="ja-tool-narrow" onclick="changeToolHilite(curtool, this);curtool=this;" /></a>
<a href="#Wide" onclick="setScreenType('wide');return false;"><img title="Wide screen" src="http://www.soltel.be/templates/ja_sargas/images/default/user-screen2-hilite.gif" alt="Wide screen resolution" id="ja-tool-wide" onclick="changeToolHilite(curtool, this);curtool=this;" /></a>
<a href="#Increase" onclick="changeFontSize(1); return false;"><img title="Increase font size" src="http://www.soltel.be/templates/ja_sargas/images/default/user-increase.gif" alt="Increase font size" id="ja-tool-increase" /></a>
<a href="#Decrease" onclick="changeFontSize(-1); return false;"><img title="Decrease font size" src="http://www.soltel.be/templates/ja_sargas/images/default/user-decrease.gif" alt="Decrease font size" id="ja-tool-decrease" /></a>
<a href="#Default" onclick="revertStyles(3); return false;"><img title="Default size" src="http://www.soltel.be/templates/ja_sargas/images/default/user-reset.gif" alt="Default font size" id="ja-tool-reset" /></a>
</span>
<script type="text/javascript">
var curtool = document.getElementById('ja-tool-wide');
</script>
<span class="ja-colorthemes">
<a href="#default" onclick="setActiveStyleSheet('default');"><img style="cursor: pointer;" src="http://www.soltel.be/templates/ja_sargas/images/default/default-hilite.gif" title="default color" alt="default color" id="ja-tool-defaultcolor" onclick="changeToolHilite(curcolor, this);curcolor=this;return false;" /></a>
<a href="#green" onclick="setActiveStyleSheet('green');"><img style="cursor: pointer;" src="http://www.soltel.be/templates/ja_sargas/images/default/green.gif" title="green color" alt="green color" id="ja-tool-greencolor" onclick="changeToolHilite(curcolor, this);curcolor=this;return false;" /></a>
<a href="#orange" onclick="setActiveStyleSheet('orange');"><img style="cursor: pointer;" src="http://www.soltel.be/templates/ja_sargas/images/default/orange.gif" title="orange color" alt="orange color" id="ja-tool-orangecolor" onclick="changeToolHilite(curcolor, this);curcolor=this;return false;" /></a>
</span>
<script type="text/javascript">
var curcolor = document.getElementById('ja-tool-defaultcolor');
</script>
<div id="ja-search">
<form action="index.php?option=com_search&Itemid=5" method="get">
<div class="search">
<input name="searchword" id="mod_search_searchword" maxlength="20" alt="search" class="inputbox" type="text" size="20" value="Rechercher..." onblur="if(this.value=='') this.value='Rechercher...';" onfocus="if(this.value=='Rechercher...') this.value='';" /> </div>
<input type="hidden" name="option" value="com_search" />
<input type="hidden" name="Itemid" value="5" />
</form> </div>
</div>
</div>
<!-- BEGIN: MAIN NAVIGATION -->
<div id="ja-mainnav">
<ul id="ja-splitmenu" class="mainlevel"><li class="active_menu"><a href="http://www.soltel.be/component/option,com_frontpage/Itemid,1/" title="Accueil"><span>Accueil</span></a></li>
<li><a href="http://www.soltel.be/content/section/1/2/" title="Articles"><span>Articles</span></a></li>
<li><a href="http://www.soltel.be/forum/" title="Forum"><span>Forum</span></a></li>
<li><a href="http://www.soltel.be/component/option,com_contact/Itemid,3/" title="Nous contacter"><span>Nous contacter</span></a></li>
<li><a href="http://www.soltel.be/component/option,com_search/Itemid,5/" title="Rechercher"><span>Rechercher</span></a></li>
</ul>
</div>
<!-- END: MAIN NAVIGATION -->
</div>
</div>
</div>
</tr>
</table>
<!-- /logo -->
<!-- content table -->
$spacer_open
$_phpinclude_output
Je te laisse le soins de modifier ce code pour l'adapter parfaitement à ton forum.