Hi if you need to change the current custom layout of the odoo. You can inherit the current external layout template in your custom module, replace the main div, and add the customization inside it 

example:

<odoo>
    <template id="inherit_external_layout"
inherit_id="web.external_layout">
        <xpath expr="//div"
               position="replace">

               // Add your custom codes here

       </xpath>

    </template>

</odoo>

Hope it helps