WordPress at a glance
function is not described

WP_Customize_Custom_CSS_Setting::update() public WP 4.7.0

Store the CSS setting value in the custom_css custom post type for the stylesheet.

{} It's a method of the class: WP_Customize_Custom_CSS_Setting{}

No Hooks.

Return

Int|false. The post ID or false if the value could not be saved.

Usage

$WP_Customize_Custom_CSS_Setting = new WP_Customize_Custom_CSS_Setting();
$WP_Customize_Custom_CSS_Setting->update( $css );
$css(string) (required)
The input value.

Changelog

Since 4.7.0 Introduced.

Code of WP_Customize_Custom_CSS_Setting::update() WP 5.7.2

<?php
public function update( $css ) {
	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;
}