body {
      margin: 0;
      font-family: sans-serif;
      background: #111;
      color: #eee;
      display: flex;
      flex-direction: column;
      height: 100vh;
    }
header, footer {
      background: #222;
      padding: 0.5em 1em;
    }
.btn {
      margin-right: 0.5em;
      background: #333;
      color: white;
      border: none;
      padding: 0.5em 1em;
      cursor: pointer;
    }
.btn.active {
      background: #555;
      color: white;
    }
main {
      flex: 1;
      display: flex;
      flex-direction: column;
      padding: 1em;
      overflow-y: auto;
    }
.page {
      display: none;
    }
.page.active {
      display: block;
    }
canvas {
      border: 1px solid #444;
    }