odoo16/addons/mass_mailing/views/themes_templates.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>
    <!-- Theme "Basic" template -->
    <template id="theme_basic_template">
        <div class="o_mail_no_options">
            <p><br/></p>
            <p><br/></p>
            <p>
                <br/>
                <a href="/unsubscribe_from_list">Unsubscribe</a>
            </p>
        </div>
    </template>

    <template id="theme_empty_template">
    </template>

    <!-- Default Theme -->
    <template id="theme_default_template">
        <style id="design-element">
            h2 {
                font-weight: bolder;
            }
            p, p > *, li, li > * {
                color: #6c757d;
            }
            hr {
                border-top-color: #ced4da !important;
            }
        </style>
        <div class="s_header_logo o_mail_block_header_logo o_mail_snippet_general pt16 pb16">
            <div class="container">
                <div class="row">
                    <div class="col-lg-4"/>
                    <div class="col-lg-4" style="text-align: center;">
                        <a t-att-href="(company_id.website) or '#'" style="text-decoration:none;" target="_blank">
                            <img border="0" src="/mass_mailing/static/src/img/theme_default/s_default_image_header_logo.png" style="height:auto; max-width:100%;" width="180"/>
                        </a>
                    </div>
                    <div class="col-lg-4" style="text-align: right;"/>
                </div>
            </div>
        </div>
        <div class="s_text_block o_mail_snippet_general pt40 pb16" style="padding-left: 15px; padding-right: 15px;">
            <div class="container s_allow_columns">
                <h2>Thank you for joining us!</h2>
                <p><br/>We want to take this opportunity to welcome you to our ever-growing community!
                <br/>Your platform is ready for work, it will help you reduce the costs of digital signatures, attract new customers and increase sales.</p>
                <p><img src="/mass_mailing/static/src/img/theme_default/signature.png" style="width:125px; margin-top:8px;margin-bottom:-25px;" alt="Signature"/></p>
                <p>Michael Fletcher<br/>
                   <span style="font-size: 12px; font-weight: bolder;">Customer Service</span>
                </p>
                <p style="text-align: center;">
                    <a role="button" href="#" class="btn btn-primary">LOGIN</a>
                </p>
            </div>
        </div>
        <div class="s_hr pt16 pb16" data-snippet="s_hr" data-name="Separator">
            <hr class="s_hr_1px s_hr_solid"/>
        </div>
        <t t-call="mass_mailing.s_mail_block_footer_social_left"/>
    </template>
</odoo>