Drop this into any website to play around with #Safari15 themed tabs!

index.html

<meta name="theme-color" content="#29B8FF" />

<input type="color" value="#29B8FF" id="colorPicker" />

<script>
  const colorPicker = document.getElementById("colorPicker")
  colorPicker.addEventListener("change", function() {
    document
      .querySelector("meta[name=theme-color]")
      .setAttribute("content", colorPicker.value)
  })
</script>