WP_Customize_Custom_CSS_Setting::update( string $css )

Source

File: wp-includes/customize/class-wp-customize-custom-css-setting.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;
	}