Here's a html e-mail template example for sending html e-mails. Want to know how to configure it? Read How to send html e-mails in drupal 8
<style type="text/css">
td {
font-family: Arial, sans-serif;
font-size: 12px;
overflow-wrap: break-word;
word-wrap: break-word;
color: #333;
}
a {
color: #000;
}
</style>
<table width="100%" cellpadding="20" cellspacing="0" bgcolor="#7EB67D">
<tr>
<td align="center" valign="top">
<div style="padding:40px 0">
<table width="600" cellpadding="0" cellspacing="0">
{% if site_logo %}
<tr>
<td align="left" valign="top" style="padding: 20px 0">
<a href="{{ base_url }}"><img src="{{ site_logo }}" alt="{{ site_name }}" border="0"/></a>
</td>
</tr>
{% endif %}
<tr>
<td align="left" valign="top" bgcolor="#ffffff" style="padding:20px">
<h3>{{ subject }}</h3>
{{ body|nl2br }}
</td>
</tr>
</table>
</div>
</td>
</tr>
</table>