Source

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

public function preview() {
		if ( $this->is_previewed ) {
			return false;
		}
		$this->is_previewed = true;
		add_filter( 'wp_get_custom_css', array( $this, 'filter_previewed_wp_get_custom_css' ), 9, 2 );
		return true;
	}