WP_Customize_Custom_CSS_Setting::update( string $value ): int|false

Source

public function update( $value ) {
	// Restores the more descriptive, specific name for use within this method.
	$css = $value;

	if ( empty( $css ) ) {
		$css = '';
	}

	$r = wp_update_custom_css_post(
		$css,
		array(
			'stylesheet' => $this->stylesheet,
		)
	);

	if ( $r instanceof WP_Error ) {
		return false;
	}
	$post_id = $r->ID;

	// Cache post ID in theme mod for performance to avoid additional DB query.
	if ( $this->manager->get_stylesheet() === $this->stylesheet ) {
		set_theme_mod( 'custom_css_post_id', $post_id );
	}

	return $post_id;
}

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