Hey;

I did as below, hope it works for you as well.

- scaffold a new module

- you need to override the style in "mass_mailing.mass_mailing_mail_style". Therefore, put the code below in the view.xml. you can find the url links for the different style of the roboto here: https://fonts.google.com/specimen/Roboto

<odoo>
  <data>
    <template id="mass_mailing_mail_style_inherited" inherit_id="mass_mailing.mass_mailing_mail_style" >
      <xpath expr="//style" position="replace">

        <style>

            .o_layout * {
                box-sizing: border-box !important;
            }

            @font-face {
                font-family: 'Roboto';
                font-style: normal;
                font-weight: 400;
                src: url('https://fonts.googleapis.com/css2?family=Roboto&display=swap'); format('truetype');
            }
            .o_layout :not(.fa) {
                    font-family: Roboto;
            }
            /* Remove space around the email design. */
                html,
                body {
                    margin: 0 auto !important;
                    padding: 0 !important;
                    height: 100% !important;
                    width: 100% !important;
                }

                /* Stop Outlook resizing small text. */
                * {
                    -ms-text-size-adjust: 100%;
                }

                /* Stop Outlook from adding extra spacing to tables. */
                table,
                td {
                    mso-table-lspace: 0pt !important;
                    mso-table-rspace: 0pt !important;
                }

                /* Use a better rendering method when resizing images in Outlook IE. */
                img {
                    -ms-interpolation-mode:bicubic;
                }

                /* Prevent Windows 10 Mail from underlining links. Styles for underlined links should be inline. */
                a {
                    text-decoration: none;
                }
            @media screen and (max-width: 768px) {
                .o_mail_table_styles {
                    width: 100% !important;
                }
            }
        style>

                xpath>
    template>
  data>
odoo>

- Add the view.xml path in your manifest file

'data': ['views/views.xml',],

All the best