// Declare $post global if used outside of the loop.
$post = get_post();

// check to see if the theme supports Featured Images, and one is set
if ( current_theme_supports( 'post-thumbnails' ) && has_post_thumbnail( $post->ID ) ) {
        
    // Specify desired image size in place of 'full'.
    $page_bg_image     = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'full' );
    $page_bg_image_url = $page_bg_image[0]; // This returns just the URL of the image.

} else {
    // The fallback – our current active theme's default bg image.
    $page_bg_image_url = get_background_image();
}

// And below, spit out the <style> tag... ?>
<style type="text/css" id="custom-background-css-override">
    body.custom-background { background-image: url('<?php echo $page_bg_image_url; ?>'); }
</style>