In the vB header template find the following table:
Code:
<table border="0" width="$stylevar[outertablewidth]" cellpadding="0" cellspacing="0" align="center">
<tr>
<td align="$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>
You can add some inline CSS to the TABLE or TD tags:
Code:
style="background: #000000;"