背景にしたいのですが・・・

以下のコードを背景にしようと試行錯誤したのですが、うまくいきませんでした。
どのようにすればいいか、教えていただけると幸いです。よろしくお願いします。

<html>
<head>

<style type="text/css">
html, body { 
  height: 100%; 
  margin: 0; 
  overflow: hidden;
}
body { 
  display: flex; 
  align-items: center; 
  justify-content: center;
  background: #000; 
}
main {
  /*display: flex;*/
}
p {
  line-height: 1;
}
span {
  display: block;
  width: 2vmax; 
  height: 2vmax; 
  font-size: 2vmax; 
  color: #9bff9b11;
  text-align: center;
  font-family: "Helvetica Neue", Helvetica, sans-serif;
}
</style>
</head>
<body>
<main></main>
</body>
<script type="text/javascript">
function r(from, to) {
  return ~~(Math.random() * (to - from + 1) + from);
}
function pick() {
  return arguments[r(0, arguments.length - 1)];
}
function getChar() {
  return String.fromCharCode(pick(
    r(0x3041, 0x30ff),
    r(0x2000, 0x206f),
    r(0x0020, 0x003f)
  ));
}
function loop(fn, delay) {
  let stamp = Date.now();
  function _loop() {
    if (Date.now() - stamp >= delay) {
      fn(); stamp = Date.now();
    }
    requestAnimationFrame(_loop);
  }
  requestAnimationFrame(_loop);
}
class Char {
  constructor() {
    this.element = document.createElement('span');
    this.mutate();
  }
  mutate() {
    this.element.textContent = getChar();
  }
}
class Trail {
  constructor(list = [], options) {
    this.list = list;
    this.options = Object.assign(
      { size: 10, offset: 0 }, options
    );
    this.body = [];
    this.move();
  }
  traverse(fn) {
    this.body.forEach((n, i) => {
      let last = (i == this.body.length - 1);
      if (n) fn(n, i, last);
    });
  }
  move() {
    this.body = [];
    let { offset, size } = this.options;
    for (let i = 0; i < size; ++i) {
      let item = this.list[offset + i - size + 1];
      this.body.push(item);
    }
    this.options.offset = 
      (offset + 1) % (this.list.length + size - 1);
  }
}
class Rain {
  constructor({ target, row }) {
    this.element = document.createElement('p');
    this.element.style.display = "flex"
    this.build(row);
    if (target) {
      target.appendChild(this.element);
    }
    this.drop();
  }
  build(row = 20) {
    let root = document.createDocumentFragment();
    let chars = [];
    for (let i = 0; i < row; ++i) {
      let c = new Char();
      root.appendChild(c.element);
      chars.push(c);
      if (Math.random() < .5) {
        loop(() => c.mutate(), r(1e3, 5e3));
      }
    }
    this.trail = new Trail(chars, { 
      size: r(10, 30), offset: r(0, 100) 
    });
    this.element.appendChild(root); 
  }
  drop() {
    let trail = this.trail;
    let len = trail.body.length;
    let delay = r(10, 100);
    loop(() => {
      trail.move();
      trail.traverse((c, i, last) => {
        c.element.style = `
          color: hsl(136, 100%, ${85 / len * (i + 1)}%)
        `;
        if (last) {
          c.mutate();
          c.element.style = `
            color: hsl(136, 100%, 85%);
            text-shadow:
              0 0 .5em #fff,
              0 0 .5em currentColor;
          `;
        }
      });
    }, delay);
  }
}

const main = document.querySelector('main');
console.log(main)
for (let i = 0; i < 50; ++i) {
  new Rain({ target: main, row: 50 });
}
</script>
</html>