and in the odoo UI directly? without code

Hi,

For this you can create a new custom module and add a new email design.

Inherit the mass_mailing.email_designer_snippets and using xpath add new template like this,

<template id="test_template" inherit_id="mass_mailing.email_designer_snippets">
    <xpath expr="//div[@data-name='default']" position="after">
        <div data-name="test_mass_mail_1"
             data-img="/module_name/static/src/img/test"
             data-images-info='{"logo": {"format": "png"}}'>
            <t t-call="module_name.theme_test"/>
        </div>
    </xpath>
</template>

Now you have to define the corresponding template,

<template id="theme_test">
   <div class="o_layout o_test_mass_mail_theme">
     <table class="o_mail_wrapper">
     </table>
   </div>
</template>

Thanks