<div class="wrap">
  <div class="in">
    Hello!
  </div>
</div>

div.wrap {
  width:512px;margin:64px auto 0;
  background: linear-gradient( to right ,
        rgba(255, 0, 0, 1) 0%,
        rgba(255, 255, 0, 1) 15%,
        rgba(0, 255, 0, 1) 30%,
        rgba(0, 255, 255, 1) 50%,
        rgba(0, 0, 255, 1) 65%,
        rgba(255, 0, 255, 1) 80%,
        rgba(255, 0, 0, 1) 100%);
  padding:2px;
  box-shadow:0px 0px 8px rgba(0, 0, 0, 0.8);
}
div.in {
  background:white;
  padding:8px;
  text-align:center;
  font-family:Arial;
  font-weight:bold;
}