<div class="card theme-new">
  <div class="card-header">
    <h2>Card Header</h2>
  </div>
</div>

.card {
  container-name: card;
  border-radius: 8px;
  box-shadow: 0 2px 1em #0004;
  padding: 1em;
}
.theme-new {
  --new: true;
}

@container card style(--new: true) {
  h2 {
    display: flex;
    align-items: center;
    gap: 0.5em;
  }
  h2:before {
    content: "NEW";
    color: #f66;
    font-size: 0.8em;
  }
}