Source

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

public function filter_previewed_wp_get_custom_css( $css, $stylesheet ) {
		if ( $stylesheet === $this->stylesheet ) {
			$customized_value = $this->post_value( null );
			if ( ! is_null( $customized_value ) ) {
				$css = $customized_value;
			}
		}
		return $css;
	}