pos/pos_cashier_login/views/assets.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright (C) 2019 by Lambda IS <https://www.lambda-is.com> -->
<odoo>

    <template id="assets" inherit_id="point_of_sale.assets">
        <xpath expr="." position="inside">
            <link rel="stylesheet" href="/pos_cashier_login/static/src/css/main.css"/>
            <script type="text/javascript" src="/pos_cashier_login/static/src/js/main.js"></script>
        </xpath>
    </template>

</odoo>