Pretty simple. Open your Admin CP -> Style Manager -> Your Style. Edit the Header Template - Look for
Code:
<a href="$vboptions[forumhome].php$session[sessionurl_q]"><img src="$stylevar[titleimage]" border="0" alt="$vboptions[bbtitle]" /></a>
and change it to
Code:
<center><a href="$vboptions[forumhome].php$session[sessionurl_q]"><img src="$stylevar[titleimage]" border="0" alt="$vboptions[bbtitle]" /></a></center>
For most this might be pretty crude and I even think that <center> is deprecated, but it works.
Cheers
OS