partner-contact/partner_street_number/views/assets.xml

<?xml version="1.0" encoding="UTF-8"?>
<odoo>

    <template id="assets_backend" name="partner_street_number backend assets" inherit_id="web.assets_backend">
        <xpath expr="." position="inside">
            <link rel="stylesheet" href="/partner_street_number/static/src/css/field.less"/>
        </xpath>
    </template>

</odoo>