Apply custom background style on an element within the body. e.g. If you're using off-canvas and on-canvas containers, you may want the background only on the #canvas. It overwrites the theme support feature array value with a custom callback, but I haven't tested what happens if other key/values are defined elsewhere. There are no filters yet f…

nested_custom_background_cb.php

function nested_custom_background_cb() {
	ob_start();
	_custom_background_cb();
	$bg_style_tag = ob_get_contents();
	ob_end_clean();
	$bg_style_tag = str_replace( 'body.custom-background', 'body.custom-background #canvas', $bg_style_tag );
	echo $bg_style_tag;
}
add_theme_support( 'custom-background', array( 'wp-head-callback' => 'nested_custom_background_cb' ) );