openproject/app/views/mailer/_notification_mailer_header.html.erb

<table width="100%" border="0" cellpadding="0" cellspacing="0" style="border-bottom: 1px solid #cccccc; margin-bottom: 30px;">
  <tr>
    <td style="width: 82px; height: 82px; vertical-align: top;">
      <%= email_image_tag("logo_openproject_narrow.svg", alt: I18n.t(:'mail.logo_alt_text')) %>
    </td>
    <td width="100%" style="padding-top: 8px;">
      <table>
        <tr>
          <td style="font-size: 24px; color: #333333; padding-bottom: 5px;">
            <%= I18n.t(:'mail.salutation', user: @user.firstname) %>
          </td>
        </tr>
        <tr>
          <td style="font-size:14px; color: #1A67A3; font-weight: bold; padding-bottom: 10px;">
            <%= summary %>
          </td>
        </tr>
        <tr>
          <td style="padding: 10px 0 25px 0;">
            <a href="<%= notifications_center_url %>"
               target="_blank"
               style="background: #D1E5F5; padding: 8px 12px; color: #1A67A3; border: 1px solid #1A67A3; border-radius: 16px; text-decoration: none;">
              <%= I18n.t(:'mail.notification.center') %>
            </a>
          </td>
        </tr>
      </table>
    </td>
  </tr>
</table>