body {
        font-family: Arial, sans-serif;
        margin: 0;
        padding: 0;
        background: #f5f5f5;
    }
    header {
        background: #2c3e50;
        color: white;
        padding: 10px 15px;
        text-align: center;
    }
    main {
        max-width: 1100px;
        margin: 20px auto;
        padding: 0 10px 30px;
    }
    h2 {
        margin-top: 0;
    }
    .section {
        background: white;
        border-radius: 8px;
        padding: 15px 20px;
        margin-bottom: 20px;
        box-shadow: 0 2px 6px rgba(0,0,0,0.1);
    }
    .number-display {
        font-size: 2rem;
        font-weight: bold;
        margin: 10px 0;
        padding: 10px 15px;
        background: #ecf0f1;
        border-radius: 6px;
        text-align: center;
        min-height: 2.5em;
    }
    .buttons {
        margin: 10px 0;
    }
    button {
        padding: 8px 14px;
        margin: 4px;
        border: none;
        border-radius: 4px;
        cursor: pointer;
        background: #3498db;
        color: white;
        font-size: 0.95rem;
    }
    button:hover {
        background: #2980b9;
    }
    button.secondary {
        background: #7f8c8d;
    }
    button.secondary:hover {
        background: #646f70;
    }
    .answer {
        margin-top: 8px;
        padding: 8px 10px;
        background: #e8f8e8;
        border-radius: 6px;
        min-height: 1.5em;
    }
    .expression {
        font-size: 1.5rem;
        text-align: center;
        background: #ecf0f1;
        padding: 10px 15px;
        border-radius: 6px;
        margin: 10px 0;
        word-wrap: break-word;
    }
    .small-note {
        font-size: 0.85rem;
        color: #555;
        margin-top: 5px;
    }