.print-heading {
	text-align: right;
	text-transform: uppercase;
	font-color: #888;
	font-weight: bold;
	padding-bottom: 7px;
	margin-bottom: 15px;
	border-bottom: 3px solid #aaa;
}

.print-format table.table-bordered {
	border-top: 3px solid #aaa;
}

.print-format th {
	background-color: #eee;
}

.print-heading {
	text-align: right;
	text-transform: uppercase;
	font-color: #888;
	font-weight: bold;
	padding-bottom: 7px;
	margin-bottom: 15px;
	border-bottom: 3px solid #aaa;
}

.print-format table.table-bordered {
	border-top: 3px solid #aaa;
}

.print-format th {
	background-color: #eee !important;
}

Fix table header background color in pdf view
Fix table header background color in pdf view

wkhtmltopdf removes div/th background color
reference: http://stackoverflow.com/questions/25118343/wkhtmltopdf-div-background-color

	background-color: #eee !important; //will fix the problem in Modern Print Pdf View

Check this url

Table Header border is comming in print priview
https://demo.erpnext.com/api/method/frappe.templates.pages.print.download_pdf?doctype=Sales%20Invoice&name=%5BSelect%5D00024&format=Standard&no_letterhead=0

but Table Header background color is removed in pdf
https://demo.erpnext.com/print?doctype=Sales%20Invoice&name=%5BSelect%5D00024&format=Standard&no_letterhead=0