web/web_group_by_percentage/templates/assets.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright 2019 Onestein
     License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl). -->

<odoo>
    <template id="assets_backend" inherit_id="web.assets_backend">
        <xpath expr=".">
            <script src="/web_group_by_percentage/static/src/js/backend.js"></script>
        </xpath>
        <xpath expr="//link[last()]" position="after">
            <link rel="stylesheet" type="text/scss" href="/web_group_by_percentage/static/src/scss/backend.scss"/>
        </xpath>
    </template>
</odoo>