web/web_editor_background_color/templates/assets.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright 2017-2019 Jairo Llopis <jairo.llopis@tecnativa.com>
     License LGPL-3.0 or later (http://www.gnu.org/licenses/lgpl.html). -->

<odoo>

    <template id="assets_editor" inherit_id="web_editor.assets_editor">
        <xpath expr=".">
            <link rel="stylesheet" href="/web_editor_background_color/static/src/css/background_color.scss"/>
            <script type="text/javascript" src="/web_editor_background_color/static/src/js/background_color.js"/>
        </xpath>
    </template>

</odoo>