would be simple to add to the forum.tpl
basically, you add something like
{if $smarty.section.fora.iteration is odd} alt-forum-bg{/if}
to the <td class="xxxx".... > elements inside the fora, section
example:
<td class="forumtitle{if $smarty.section.fora.iteration is odd} alt-forum-bg{/if}" valign="top" align="left">
do that for each column td and bada bing..
then add the .alt-forumlist-bg class to style the alternate BG background-color:#xxx; in the Extra CSS section