It seems that there is something odd about your site setup. Did you upgrade the license from X to Pro? If you have, please check if the child theme installed is updated since the older versions of the child theme is slightly different from the updated ones.
Please login through FTP then go to wp-content/pro-child then check and edit the functions.php file if necessary. It should have this basic lines:
<?php
// =============================================================================
// FUNCTIONS.PHP
// -----------------------------------------------------------------------------
// Overwrite or add your own custom functions to Pro in this file.
// =============================================================================
// =============================================================================
// TABLE OF CONTENTS
// -----------------------------------------------------------------------------
// 01. Enqueue Parent Stylesheet
// 02. Additional Functions
// =============================================================================
// Enqueue Parent Stylesheet
// =============================================================================
add_filter( 'x_enqueue_parent_stylesheet', '__return_true' );
// Additional Functions
// =============================================================================
Once everything is correctly setup, this code should remove the featured image on the blog and single posts:
CSS is not working because default wordpress class added on the body is missing. Either it was removed in the child theme or the main Pro theme is corrupted. Please try to switch to main Pro theme. Check if that class will work, if yes, you might have a customization that removes that class. If not, please try reinstalling Pro theme. Purge cache after this and then try again.
I checked your site and the problem was caused by this plugin Multi Form Data Storage, if you will deactivate the plugin the custom css that you have added will take effect.
But if you need the Multi Form Data Storage plugin and you can’t find an alternative plugin for it, I have another solution that will not involve deactivating it. Just add this code below in the functions.php of your child theme.