WordPress at a glance
function is not described

_custom_background_cb() WP 3.0.0

Default custom background callback.

This is an internal function for using it by WP core itself. It's not recommended to use this function in your code.

No Hooks.

Return

null. Nothing.

Usage

_custom_background_cb();

Changelog

Since 3.0.0 Introduced.

Code of _custom_background_cb() WP 5.7.2

<?php
function _custom_background_cb() {
	// $background is the saved custom image, or the default image.
	$background = set_url_scheme( get_background_image() );

	// $color is the saved custom color.
	// A default has to be specified in style.css. It will not be printed here.
	$color = get_background_color();

	if ( get_theme_support( 'custom-background', 'default-color' ) === $color ) {
		$color = false;
	}

	$type_attr = current_theme_supports( 'html5', 'style' ) ? '' : ' type="text/css"';

	if ( ! $background && ! $color ) {
		if ( is_customize_preview() ) {
			printf( '<style%s id="custom-background-css"></style>', $type_attr );
		}
		return;
	}

	$style = $color ? "background-color: #$color;" : '';

	if ( $background ) {
		$image = ' background-image: url("' . esc_url_raw( $background ) . '");';

		// Background Position.
		$position_x = get_theme_mod( 'background_position_x', get_theme_support( 'custom-background', 'default-position-x' ) );
		$position_y = get_theme_mod( 'background_position_y', get_theme_support( 'custom-background', 'default-position-y' ) );

		if ( ! in_array( $position_x, array( 'left', 'center', 'right' ), true ) ) {
			$position_x = 'left';
		}

		if ( ! in_array( $position_y, array( 'top', 'center', 'bottom' ), true ) ) {
			$position_y = 'top';
		}

		$position = " background-position: $position_x $position_y;";

		// Background Size.
		$size = get_theme_mod( 'background_size', get_theme_support( 'custom-background', 'default-size' ) );

		if ( ! in_array( $size, array( 'auto', 'contain', 'cover' ), true ) ) {
			$size = 'auto';
		}

		$size = " background-size: $size;";

		// Background Repeat.
		$repeat = get_theme_mod( 'background_repeat', get_theme_support( 'custom-background', 'default-repeat' ) );

		if ( ! in_array( $repeat, array( 'repeat-x', 'repeat-y', 'repeat', 'no-repeat' ), true ) ) {
			$repeat = 'repeat';
		}

		$repeat = " background-repeat: $repeat;";

		// Background Scroll.
		$attachment = get_theme_mod( 'background_attachment', get_theme_support( 'custom-background', 'default-attachment' ) );

		if ( 'fixed' !== $attachment ) {
			$attachment = 'scroll';
		}

		$attachment = " background-attachment: $attachment;";

		$style .= $image . $position . $size . $repeat . $attachment;
	}
	?>
<style<?php echo $type_attr; ?> id="custom-background-css">
body.custom-background { <?php echo trim( $style ); ?> }
</style>
	<?php
}