mail-addons/mail_to/templates.xml

<?xml version="1.0"?>
<openerp>
    <data>
        <template id="mail_to_assets_backend"
                  name="mail_to_assets_backend"
                  inherit_id="web.assets_backend">
            <xpath expr="." position="inside">
                <link rel="stylesheet" href="/mail_to/static/src/css/mail_to.css"/>
            </xpath>
        </template>
    </data>
</openerp>