Pressbooks\Modules\Export\Mpdf\Pdf::setCss PHP Method

    function setCss()
    {
        $css = '';
        // check for child theme export file
        $cssfile = $this->getExportStylePath('mpdf');
        // if empty, try the parent theme export directory
        if (empty($cssfile)) {
            $cssfile = realpath(get_template_directory() . '/export/mpdf/style.css');
        }
        if (is_string($cssfile) && !empty($cssfile)) {
            $css .= file_get_contents($cssfile) . "\n";
        }
        // grab the web theme, ONLY as a backup
        if (empty($css)) {
            $theme = wp_get_theme();
            $css = $this->getThemeCss($theme);
        }
        // Theme options override
        $css .= apply_filters('pb_mpdf_css_override', $css) . "\n";
        if (!empty($css)) {
            $this->mpdf->WriteHTML($css, self::MODE_CSS);
        }
    }