Your javascript function is opening a new window, and creating a new document which doesn't contain any link to your stylesheet.
Update your function so that it adds the relevant stylesheet link to the new document:
function PrintPanel() {
var panel = document.getElementById("<%=panelprint.ClientID %>");
var printWindow = window.open('', '', '');
printWindow.document.write('<html><head><title>Print Invoice</title>');
printWindow.document.write('<base href="' + location.origin + location.pathname + '">');
printWindow.document.write('<link rel="stylesheet" href="css/invoice.css">');
printWindow.document.write('<style type="text/css">.style1{width: 100%;}</style>');
printWindow.document.write('</head><body >');
printWindow.document.write(panel.innerHTML);
printWindow.document.write('</body></html>');
printWindow.document.close();
setTimeout(function () {
printWindow.print();
}, 500);
return false;
}</base>