private function addOpeningNav(string $id): string
    {
        $output = '<nav ';
        if ($id !== '') $output .= 'id="' . $id . '" ';
        $output .= 'class="navbar navbar-' . $this->options['theme']['fore'];
        if (!empty($this->options['theme']['back'])) $output .= " bg-" . $this->options['theme']['back'];
        if ($this->options['mobile']) {
            $output .= ' navbar-expand-' . $this->options['mobileSize'];
        } else {
            $output .= ' navbar-expand';
        }
        if (!empty($this->options['theme']['color'])) $output .= '" style="background-color: #' . $this->options['theme']['color'];
        $output .= '">';
        return $output;
    }