Try starting with the template "FORUMHOME" (perhaps "FORUMDISPLAY" also), you will see:
HTML Code:
<!-- main -->
<table class="tborder" cellpadding="$stylevar[cellpadding]" cellspacing="$stylevar[cellspacing]" border="0" width="100%" align="center">
<thead>
<tr align="center">
<td class="thead"> </td>
<td class="thead" width="100%" align="$stylevar[left]">$vbphrase[forum]</td>
You can see that the cellpadding and cellspacing are defined by stylevars there. The first thing I would try is go to your style and the drop-down to StyleVars. There you will see the corresponding stylevars for the cellspacing and cellpadding:
Inner Border Width
Table Cell Padding (pixels)
Change it there first and see if you like the look on the the whole forum.