Gradientes

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>CSS Gradiente</title>
  <style>
    div {
      border: 2px solid black;
      width: 200px;
      height: 200px;
      margin: 20px auto;
    }

    .quadrado {
      background: linear-gradient(#4158D0, #C850C0, #FFCC70);
    }

    .bolinha {
      border-radius: 50%;
      background-image: radial-gradient(cyan, violet, cyan);
    }
  </style>
</head>

<body>

  <div class="quadrado"></div>
  <div class="bolinha"></div>

</body>

</html>