JSFiddle - React, Tailwind, and code Playground

HTML

<div class="cool-border">
  <div class="big-white-area">
    
  </div>
</div>

CSS

.cool-border {
  width: 256px;
  height: 46px;
  padding: 1px;
  background: linear-gradient( 90deg, #419cfd, #00a9fe, #00b4f7, #00bde9, #00c5d6, #00cac1, #38ceac, #6ed199);
  box-sizing: border-box;
}

.big-white-area {
  height: 100%;
  width: 100%;
  background-color: white;
  box-sizing: border-box;
}