Hi john47,
You can add the following file to the \templates\t4_bs5_blank\html\layouts\t4 folder:
And now, open the index.php file and custom the <body> tag.
NOTE: I have tried with this code:
<?php
// Get the current item ID
$itemId = JFactory::getApplication()->input->getInt('Itemid');
// Get the parent ID of the current item
$db = JFactory::getDbo();
$query = $db->getQuery(true)
->select('parent_id')
->from($db->quoteName('#__menu'))
->where($db->quoteName('id') . ' = ' . $db->quote($itemId));
$db->setQuery($query);
$parentId = $db->loadResult();
// Output the parent_id in the body tag
?>
<body class="{t4post:bodyclass} item-<?php echo $parentId; ?>" data-jver="{t4post:jversion}">