Flatshop: use the product image as the background image when no background image is specified.

gistfile1.php

<?php

function custom_product_default_bg_image() {
    $bg_image = themify_get( 'background_image' );
	if( ! $bg_image ) {
		global $post;
		$thumb_url = wp_get_attachment_image_src( get_post_thumbnail_id(),'shop_single', true );
		echo "<style> .post-{$post->ID} .product-bg { background-image: url('{$thumb_url[0]}'); } </style>";
	}
}
add_action( 'woocommerce_after_shop_loop_item', 'custom_product_default_bg_image' );