How to customize template style?

To change the styles (color, size, etc) of the Challan Invoice templates, you have to use the action hook woo_invoice_custom_style. See the example below about how to change the background color of the invoice: 

add_action('woo_invoice_custom_style', 'woo_invoice_custom_background');
function woo_invoice_custom_order_number_style() {
    ?>

    body{
       background-color: #c2c7ce;
    }

    <?php
}

To make the order number bold, see the example below:

add_action('woo_invoice_custom_style', 'woo_invoice_custom_order_number_style');
function woo_invoice_custom_order_number_style() {
    ?>
	
    .order-data-table tbody tr:nth-child(2) td{
        font-weight: bold;
    }

    <?php
}