pos/pos_hide_banknote_button/views/assets.xml

<?xml version="1.0" encoding="utf-8"?>
<!--
    Copyright (C) 2020 - Today: GRAP (http://www.grap.coop)
    @author: Sylvain LE GAL (https://twitter.com/legalsylvain)
    License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
-->

<odoo>

    <template id="index" inherit_id="point_of_sale.index">
        <xpath expr="//head" position="inside">
            <link rel="stylesheet" href="/pos_hide_banknote_button/static/src/css/pos_hide_banknote_button.css"/>
        </xpath>
    </template>

</odoo>