You can apply style different sections of invoice by setting style of that the section. You can set this style information directly in data file or using the function setInvoiceStyleSettings().
$xinvoice = new Xinvoice();
/**
* Set style settings of invoice sections
* @param string $section section of invoice
* @param string $subsection sub section of invoice (optional)
* @param bool $style set style of section true to show, false to style
* return object Object of xinvoice
*/
// style complete header"
$xinvoice->setInvoiceStyleSettings("header","","background: #ccc");
//
$xinvoice->setInvoiceStyleSettings("invoice","date","font-weight:bold");
echo $xinvoice->render();