Use Theme Background Image as Fallback if No Featured Image Exists

This example could be used to detect whether the current Page/Post has a Featured Image set – if so, it will use the Featured Image as the page background, if not it will use the current active theme’s default background image. As is, this should be used in the of the page template, just after the call to wp_head():

// 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>