web_techsystech/web_widget_mail_send_odoo/assets.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>

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

    </data>
</openerp>