okay- what you need to do is find tpl_tabular_display.php (it's in includes/templates/template_default/common). Copy this to your custom folder, common subfolder. This is the file that creates the tabled display of products. Change this to include your rounded boxes (as per the HTML that you created).
Then find the following code in your custom tpl_main_page.php
PHP Code:
<?php
/**
* prepares and displays center column
*
*/
echo $body_code;
require($body_code); ?>
Either add the following line after the code shown or replace the code shown with:
PHP Code:
<?php require($template->get_template_dir('tpl_tablular_display.php',DIR_WS_TEMPLATE, $current_page_base,'common'). '/tpl_tabular_display.php'); ?>