* {
      margin: 0;
      padding: 0;
      box-sizing: border-box;
    }

    body {
      font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
      background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
      min-height: 100vh;
      display: flex;
      justify-content: center;
      align-items: center;
      padding: 20px;
    }

    .calculator-container {
      background: rgba(255, 255, 255, 0.1);
      backdrop-filter: blur(20px);
      border-radius: 30px;
      padding: 30px;
      box-shadow: 0 20px 60px rgba(0, 0, 0, 0.3),
                  inset 0 0 20px rgba(255, 255, 255, 0.1);
      border: 2px solid rgba(255, 255, 255, 0.2);
      max-width: 400px;
      width: 100%;
    }

    .header {
      text-align: center;
      margin-bottom: 20px;
    }

    .header h2 {
      color: #fff;
      font-size: 1.8rem;
      margin-bottom: 5px;
      text-shadow: 0 2px 10px rgba(0, 0, 0, 0.3);
    }

    .header p {
      color: rgba(255, 255, 255, 0.8);
      font-size: 0.9rem;
      font-style: italic;
    }

    .display-container {
      background: rgba(0, 0, 0, 0.3);
      border-radius: 20px;
      padding: 20px;
      margin-bottom: 20px;
      border: 2px solid rgba(255, 255, 255, 0.1);
      box-shadow: inset 0 2px 10px rgba(0, 0, 0, 0.3);
    }

    .display {
      background: transparent;
      border: none;
      color: #fff;
      font-size: 2.5rem;
      text-align: right;
      width: 100%;
      outline: none;
      font-weight: 300;
      letter-spacing: 2px;
      text-shadow: 0 0 10px rgba(255, 255, 255, 0.5);
    }

    .history {
      color: rgba(255, 255, 255, 0.6);
      font-size: 1rem;
      text-align: right;
      min-height: 20px;
      margin-bottom: 10px;
    }

    .buttons {
      display: grid;
      grid-template-columns: repeat(4, 1fr);
      gap: 12px;
    }

    button {
      padding: 20px;
      font-size: 1.3rem;
      border: none;
      border-radius: 15px;
      cursor: pointer;
      transition: all 0.3s;
      font-weight: 600;
      color: #fff;
      background: rgba(255, 255, 255, 0.1);
      border: 2px solid rgba(255, 255, 255, 0.2);
      box-shadow: 0 4px 15px rgba(0, 0, 0, 0.2);
    }

    button:hover {
      transform: translateY(-3px);
      box-shadow: 0 6px 20px rgba(0, 0, 0, 0.3);
      background: rgba(255, 255, 255, 0.2);
    }

    button:active {
      transform: translateY(0);
      box-shadow: 0 2px 10px rgba(0, 0, 0, 0.2);
    }

    .btn-number {
      background: rgba(255, 255, 255, 0.15);
    }

    .btn-operator {
      background: linear-gradient(135deg, #667eea, #764ba2);
      font-size: 1.5rem;
    }

    .btn-operator:hover {
      background: linear-gradient(135deg, #764ba2, #667eea);
    }

    .btn-clear {
      background: linear-gradient(135deg, #f093fb, #f5576c);
      grid-column: span 2;
    }

    .btn-clear:hover {
      background: linear-gradient(135deg, #f5576c, #f093fb);
    }

    .btn-equals {
      background: linear-gradient(135deg, #4facfe, #00f2fe);
      grid-column: span 2;
      font-size: 1.8rem;
    }

    .btn-equals:hover {
      background: linear-gradient(135deg, #00f2fe, #4facfe);
    }

    .learning-note {
      margin-top: 20px;
      padding: 15px;
      background: rgba(255, 255, 255, 0.1);
      border-radius: 15px;
      border-left: 4px solid #4facfe;
    }

    .learning-note p {
      color: rgba(255, 255, 255, 0.9);
      font-size: 0.85rem;
      line-height: 1.6;
    }

    .learning-note strong {
      color: #4facfe;
    }

    @media (max-width: 480px) {
      .calculator-container {
        padding: 20px;
      }

      .display {
        font-size: 2rem;
      }

      button {
        padding: 15px;
        font-size: 1.1rem;
      }
    }