One shortcoming of the standard template coding is that there is no way to address the center <td> element uniquely. If you edit /includes/templates/your_template/common/tpl_main_page.php in this area:
PHP Code:
<div id="navColumnOneWrapper" style="width: <?php echo BOX_WIDTH_LEFT; ?>"><?php require(DIR_WS_MODULES . zen_get_module_directory('column_left.php')); ?></div></td>
<?php
}
?>
<td valign="top">
to add an id to the <td>, you can style the entire center section in your stylesheet.
PHP Code:
<div id="navColumnOneWrapper" style="width: <?php echo BOX_WIDTH_LEFT; ?>"><?php require(DIR_WS_MODULES . zen_get_module_directory('column_left.php')); ?></div></td>
<?php
}
?>
<td id="centerColumn" valign="top">
Code:
#centerColumn {background-color: #aabbcc;}