You'd edit the template:
ubbt_unregisterednav.tmpl
Editing templates do not invalidate your support agreement.
Change this section:
·
<a href="$phpurl/newuser.php?Cat=$Cat" target="_top">{$ubbt_lang['NEW_USER']}</a>
&middot;
to something like:
&middot;
<b><a href="$phpurl/newuser.php?Cat=$Cat" target="_top">{$ubbt_lang['NEW_USER']}</a></b>
&middot;
That would make the new user link
bold