I wrote new qweb report for Sale Order. 

<template id="report_saleorder_approval_view">

<t t-call="report.html_container">

   <t t-set="data_report_margin_top" t-value="12"/>

   <t t-set="data_report_header_spacing" t-value="9"/>

   <t t-set="data_report_dpi" t-value="110"/>

   <t t-foreach="docs" t-as="o">

   <t t-call="report.external_layout">

       <div class="page">

           <div class="oe_structure"/           

           <div class="text-center"><p style="font-size:14px;"><b><u>Approval View</u></b></p></div >

<div class="row mt32 mb32" style="font-size:14px;">

<div class="col-xs-offset-0 col-xs-4">

<span class="text-left" t-if="o.name_confirm">Our Ref : <b><span t-field="o.name"/></b></span>

</div

</div>

<div style="font-size:10px;">

   <table class="table table-bordered"  style="border: 2px solid black;">

       <thead>

           <tr style="border: 2px solid black; font-size:10px;">

            <th class="text-center" style="border: 1px solid black; background-color:#CC0000;">Product Description</th>

               <th class="text-center" style="border: 1px solid black; background-color:#CC0000;">Quantity</th>

               <th class="text-center" style="border: 1px solid black; background-color:#CC0000;">Rate</th>

               <th class="text-center" style="border: 1px solid black; background-color:#CC0000;">Discount in Percentage</th>

               <th class="text-center" style="border: 1px solid black; background-color:#CC0000;">Discount in Value</th>

               <th class="text-center" style="border: 1px solid black; background-color:#CC0000;">Rate after Discount</th>

               <th class="text-center" style="border: 1px solid black; background-color:#CC0000;">Amount</th>

           </tr>

       </thead>

       <tbody>

        <tr style="border: 2px solid black; page-break-inside: avoid" t-foreach="o.order_line" t-as="l">

        <td style="border: 2px solid black; font-size:9px;"><span t-field="l.product_id.name"/></td>

               <td class="text-right" style="border: 2px solid black;"><span t-field="l.product_uom_qty"/></td>

               <td class="text-right" style="border: 2px solid black;"><span t-field="l.price_unit"/></td>

               <td class="text-right" style="border: 2px solid black;"><span t-field="l.discount"/></td>

               <td class="text-right" style="border: 2px solid black;"><span t-esc="get_discount_in_value(l)"/></td>

               <td class="text-right" style="border: 2px solid black;"><span t-field="l.price_subtotal"/></td>

               <td class="text-right" style="border: 2px solid black;"><span t-esc="get_amount(l)"/></td>

        </tr>

       </tbody>

       </table>

    </div>

                                </t>

</t>

</t>

</template>

 In table header, I need to appear background color so I used background-color:#CC0000; in style in the table header tag. But its not working. And I also tried bgcolor="#CC0000". This also not working.