body {
            font-family: 'Arial', sans-serif;
            display: flex;
            justify-content: center;
            align-items: center;
            min-height: 100vh;
            background-color: #e0f7fa; /* Light teal background */
            margin: 0;
            padding: 20px;
            box-sizing: border-box;
            color: #333;
        }

        .container {
            background-color: #ffffff;
            border-radius: 15px;
            box-shadow: 0 10px 30px rgba(0, 0, 0, 0.1);
            padding: 35px;
            max-width: 1200px;
            width: 100%;
            text-align: center;
            animation: fadeIn 0.8s ease-out;
        }

        @keyframes fadeIn {
            from { opacity: 0; transform: translateY(-20px); }
            to { opacity: 1; transform: translateY(0); }
        }

        h1 {
            color: #00796b; /* Dark teal heading */
            margin-bottom: 25px;
            font-size: 2.5em;
            position: relative;
            padding-bottom: 10px;
        }
        h1::after {
            content: '';
            position: absolute;
            left: 50%;
            bottom: 0;
            transform: translateX(-50%);
            width: 80px;
            height: 4px;
            background-color: #00bcd4; /* Cyan accent */
            border-radius: 2px;
        }

        /* How to Use Instructions */
        .instructions {
            background-color: #e0f2f7; /* Lighter blue for instructions */
            border: 1px solid #b2ebf2;
            border-radius: 8px;
            padding: 15px 20px;
            margin-bottom: 25px;
            text-align: left;
            font-size: 0.9em;
            color: #006064; /* Darker teal text */
            line-height: 1.5;
        }
        .instructions h3 {
            margin-top: 0;
            color: #00796b;
            font-size: 1.2em;
            margin-bottom: 10px;
        }
        .instructions ul {
            margin: 0;
            padding-left: 20px;
        }
        .instructions li {
            margin-bottom: 5px;
        }

        .input-group {
            margin-bottom: 20px;
            text-align: left;
        }

        .input-group label {
            display: block;
            margin-bottom: 8px;
            font-weight: bold;
            color: #555;
            font-size: 1em;
        }

        .input-group input[type="number"] {
            width: calc(100% - 20px);
            padding: 12px 10px;
            border: 1px solid #ccc;
            border-radius: 6px;
            font-size: 1.1em;
            box-sizing: border-box;
            transition: border-color 0.3s ease, box-shadow 0.3s ease;
        }

        .input-group input[type="number"]:focus {
            border-color: #00bcd4;
            outline: none;
            box-shadow: 0 0 0 3px rgba(0, 188, 212, 0.2);
        }

        .unit-toggle {
            display: flex;
            justify-content: center;
            margin-bottom: 25px;
            background-color: #f0f0f0;
            border-radius: 8px;
            padding: 5px;
        }

        .unit-toggle button {
            flex: 1;
            padding: 10px 15px;
            border: none;
            border-radius: 6px;
            background-color: transparent;
            font-size: 1em;
            cursor: pointer;
            transition: background-color 0.3s ease, color 0.3s ease;
            color: #555;
        }

        .unit-toggle button.active {
            background-color: #00bcd4; /* Cyan when active */
            color: white;
            font-weight: bold;
            box-shadow: 0 2px 5px rgba(0, 0, 0, 0.1);
        }

        .btn-calculate {
            padding: 15px 30px;
            background-color: #00796b; /* Dark teal button */
            color: white;
            border: none;
            border-radius: 8px;
            font-size: 1.2em;
            cursor: pointer;
            transition: background-color 0.3s ease, transform 0.2s ease;
            text-transform: uppercase;
            font-weight: bold;
            letter-spacing: 0.5px;
            width: 100%;
        }

        .btn-calculate:hover {
            background-color: #004d40; /* Even darker teal on hover */
            transform: translateY(-2px);
        }

        .result-output {
            margin-top: 30px;
            padding: 20px;
            background-color: #f2f7fa; /* Lighter background for results */
            border: 1px solid #c8e6c9; /* Light green border */
            border-radius: 8px;
            text-align: center;
            min-height: 120px;
            display: flex;
            flex-direction: column;
            justify-content: center;
            align-items: center;
            animation: slideInUp 0.5s ease-out;
        }

        @keyframes slideInUp {
            from { opacity: 0; transform: translateY(20px); }
            to { opacity: 1; transform: translateY(0); }
        }

        .result-output p {
            margin: 5px 0;
            font-size: 1.1em;
            color: #444;
        }

        .result-output #bmiValue {
            font-size: 2.2em;
            font-weight: bold;
            color: #00796b; /* Dark teal for BMI value */
            margin-bottom: 10px;
        }

        .result-output #bmiCategory {
            font-size: 1.4em;
            font-weight: bold;
            padding: 5px 15px;
            border-radius: 5px;
            display: inline-block; /* To apply background and padding */
        }

        /* Category specific colors */
        .category-underweight { background-color: #ffcdd2; color: #d32f2f; } /* Light Red */
        .category-normal { background-color: #c8e6c9; color: #388e3c; }    /* Light Green */
        .category-overweight { background-color: #ffe0b2; color: #f57c00; } /* Light Orange */
        .category-obese { background-color: #ffab91; color: #e64a19; }     /* Darker Orange/Red */
        .category-error { background-color: #ffcdd2; color: #d32f2f; }     /* Red for error */

        /* Responsive adjustments */
        @media (max-width: 480px) {
            .container {
                padding: 20px;
            }
            h1 {
                font-size: 2em;
            }
            .btn-calculate {
                font-size: 1.1em;
                padding: 12px 20px;
            }
            .result-output #bmiValue {
                font-size: 1.8em;
            }
            .result-output #bmiCategory {
                font-size: 1.2em;
            }
        }