pos/pos_mail_receipt/templates/assets.xml

<?xml version="1.0" encoding="utf-8"?>

<odoo>

    <template id="assets" inherit_id="point_of_sale.assets">
        <xpath expr=".">
            <link href="/pos_mail_receipt/static/src/css/pos.css" rel="stylesheet"/>
            <script
                type="text/javascript"
                src="/pos_mail_receipt/static/src/js/screens.js"/>
        </xpath>
    </template>

     <template id="report_assets_pdf" inherit_id="web.report_assets_pdf">
         <xpath expr=".">
            <link href="/pos_mail_receipt/static/src/css/print.css" rel="stylesheet"/>
        </xpath>
    </template>

</odoo>