[Chris M]

You will need to create a repeating image...

Then add this css:

.fluidheader {
background: #hexcode url('/path.to.repeat/image.ext') repeat-x;
}

Now alter these lines of the header template:

<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]"> 
        &nbsp; 
    </td>

Replace them with:

<td width="100%" class="fluidheader" style="border: #000000 1px solid;"><a href="$vboptions[forumhome].php$session[sessionurl_q]"><img src="$stylevar[titleimage]" border="0" alt="$vboptions[bbtitle]" /></a></td>

If memory serves (I may have gotten a few bits wrong ) this should create a 100% width table, with your header image on the absolute left, with the repeating image repeating from the end of the banner to the far right, and the whole lot encased in a 1px thick black border

Chris