To add your custom css file, you need to add the file in you custom view.

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