JSFiddle - React, Tailwind, and code Playground

HTML

<span>18</span>

CSS

body {
  background: #000;
}

span {
  position: relative;
  width: 50px;
  height: 24px;
  margin: 30px;
  border-radius: 30px;
  background: #fff;
  display: flex;
  justify-content: center;
  align-items: center;
  background: #000;
  color: #fff;
  
}

span::before {
  content: "";
  position: absolute;
  top: -2px;
  bottom: -2px;
  left: -2px;
  right: -2px;
  background: linear-gradient(90deg, rgba(38, 255, 242, 1) 0%, rgba(50, 108, 255, 1) 35%);
  border-radius: 34px;
  z-index: -1;
}