I have code using Jopmla 3 (used in https://www.stmaryscalne.org/index.php/academic) written below and it enables me to style pages according to their id, category or parent category. I used it for having different banner images for different categories.

Unfortunately it doesn't work with Joomla 4 as some at least of the code is deprecated. Any help appreciated.

<!-- John's script for body tags -->	
<?php
  $id = JRequest::getInt('id');
  $view = JRequest::getCmd('view', 0);
  $db = &JFactory::getDBO();
  $db->setQuery('SELECT catid FROM #__content  WHERE id='.$id);   
  $cat_id = $db->loadResult();

// if (JRequest::getInt('id') == ('1774') ) :``
if ($cat_id >= ('0') ) :
{
$db->setQuery('SELECT parent_id FROM #__categories WHERE id='.$cat_id);

$parent_cat_id = $db->loadResult();
}
endif;
if ($cat_id == '' ) :
{
$parent_cat_id = 'blank';
$cat_id = 'blank';
}
endif;
?>


<body class = " {t4post:bodyclass} parent_cat_id_<?php echo $parent_cat_id; ?> cat_id_<?php echo $cat_id; ?> menu_id_<?php echo JRequest::getInt('Itemid'); ?> article_id_<?php echo JRequest::getInt('id'); ?>    view_<?php echo $view; ?>   "> 

RESULT!

<body class = " site-home-slider-only navigation-default theme-default layout-default-slider-only nav-breakpoint-lg com_content view-article item-1698 parent_cat_id_7 cat_id_8 menu_id_1698 article_id_8    view_article   ">