web/web_listview_custom_element_number/views/templates.xml

<?xml version="1.0" encoding="UTF-8"?>
<!--
Copyright (C) 2015-Today Akretion (http://www.akretion.com)
@author: Sylvain LE GAL (https://twitter.com/legalsylvain)
License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
-->

<odoo>

    <template id="assets_backend" name="web_listview_custom_element_number assets" inherit_id="web.assets_backend">
        <xpath expr="." position="inside">
            <link rel="stylesheet" href="/web_listview_custom_element_number/static/src/css/web_listview_custom_element_number.css"/>
            <script type="text/javascript" src="/web_listview_custom_element_number/static/src/js/web_listview_custom_element_number.js"></script>
        </xpath>
    </template>

</odoo>