Hi,

To change the font family in an email template in Odoo 16, you can directly include inline or embedded CSS in the template's HTML. Here's a simple example and a few tips to help you customize it correctly.

<t t-name="custom.email_template">

  <div style="font-family: 'Arial', sans-serif; font-size: 14px; color: #333;">

    <p>Hello <t t-esc="object.name"/>!</p>

    <p>Thanks for contacting us.</p>

  </div>

</t>

Hope it helps