function vendor_invoice_enqueue_styles() {
    wp_enqueue_style(
        'vendor-invoice-style', // Handle   
        plugins_url( 'wc-frontend-manager-ultimate/views/vendor_invoice/style.css' ),
        array(),               // keine Abhängigkeiten
        '1.0.0',               // Version
        'all'                  // Media-Typ
    );
}
add_action( 'wp_enqueue_scripts', 'vendor_invoice_enqueue_styles' );

<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/xsl" href="https://lubeckdutyfree.com/wp-sitemap-index.xsl" ?>
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