This may or may not be a tough one ;)
First you could try taking the display of the right column out which may automatically widen the center column. To do this either find #navColumnTwo{} in the style sheet and set it to #navColumnTwo{display:none}.
If you know your never going to use the right column you could take it right out of the code by opening includes/templates/template_default/common/tpl_main_page.php and removing:
<?php
//if (COLUMN_RIGHT_STATUS == 0 || (CUSTOMERS_APPROVAL == '1' and $_SESSION['customer_id'] == '') || (CUSTOMERS_APPROVAL_AUTHORIZATION == 1 && CUSTOMERS_AUTHORIZATION_COLUMN_RIGHT_OFF == 'true' && $_SESSION['customers_authorization'] != 0)) {
if (COLUMN_RIGHT_STATUS == 0 || (CUSTOMERS_APPROVAL == '1' and $_SESSION['customer_id'] == '') || (CUSTOMERS_APPROVAL_AUTHORIZATION == 1 && CUSTOMERS_AUTHORIZATION_COLUMN_RIGHT_OFF == 'true' and ($_SESSION['customers_authorization'] != 0 or $_SESSION['customer_id'] == ''))) {
// global disable of column_right
$flag_disable_right = true;
}
if (!isset($flag_disable_right) || !$flag_disable_right) {
?>
<td id="navColumnTwo" class="columnRight" style="width: <?php echo COLUMN_WIDTH_RIGHT; ?>">
<?php
/**
* prepares and displays right column sideboxes
*
*/
?>
<div id="navColumnTwoWrapper" style="width: <?php echo BOX_WIDTH_RIGHT; ?>"><?php require(DIR_WS_MODULES . zen_get_module_directory('column_right.php')); ?></div></td>
<?php
}
?>
If you could save the tpl_main_page.php to includes/templates/YOUR_CUSTOM_FOLDER/common
Otherwise save it to the same location, but realize that future updates to Zen Cart will over write the default template files.
This way may work better then the display:none technique of the stylesheet.
After doing one of these two, if the center column doesn't expand automatically, you may need to create a custom <div> tag to wrap around the center column and adjust it accordingly in the style sheet.
Post a reply if the center column doesn't fix up after removing the right column.