app/javascript/themes/current/stylesheets/components/emails.scss
.trix-email-preview {
div,
h1,
h2,
h3,
h4,
h5,
h6,
blockquote,
a {
font-size: 0.875rem !important;
}
}
.email-attachment {
background-color: #f3f3f3;
border-radius: 5px;
padding: 4px 8px;
margin: 8px;
display: inline-block;
}