1. Create a global library item, no background, padding 0 and with bottom border.
2. Add the below code to functions.php to inject it.
3. Replace the 9999 with the post ID of the global item (you see it in the url when you edit the item).
4. You need to change something in the customizer and save before you see it.

// BOTTOM BORDER: adds the 'Section with bottom border' from the library after the main content
function hws_add_bottom_border() {
	echo do_shortcode('[et_pb_section global_module="9999"][/et_pb_section]');
}
add_action( 'et_after_main_content', 'hws_add_bottom_border' );
Code language: PHP (php)