Customised colours - Panel example

index.html

<!-- This code example is not the same as GOV.UK Frontend, and is just for demonstration -->
<style>
  .panel {
    box-sizing: border-box;

    height: 100px;
    width: 100px;
    border: 3px solid black;
    background: black;
    color: white;

    /* Center text for demo */
    text-align: center;
    line-height: 100px;
  }
</style>
<div class="panel">
  Panel
</div>