mj-style.mjml

<mjml>
  <mj-head>
    <mj-style>
      .red-text div {
        color: red !important;
        text-decoration: underline !important;
      }
    </mj-style>
  </mj-head>
  <mj-body>
    <mj-section>
      <mj-column>
        <mj-text css-class="red-text">I'm red and underlined</mj-text>
      </mj-column>
    </mj-section>
  </mj-body>
</mjml>