<section id="design-04">

    <div>
      <p class="underLine-01">下線のつけ方１</p>
      <p>下線のつけ方２</p>
      <p>下線のつけ方３</p>
    </div>

 </section>

#design-04 .underLine-01 {
  padding-bottom: 5px;
  background: linear-gradient(transparent 60%, #fff29e 40%);
}