javascript blink background color

<div class="container">
        <h1>Background Blink Effect</h1>
        <div class="controls">
            <button id="startBtn">Start Blinking</button>
            <button id="stopBtn" class="stop">Stop Blinking</button>
        </div>
        <div class="info">
            Click "Start Blinking" to see the background color animation
        </div>
    </div>

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

        body {
            font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
            min-height: 100vh;
            display: flex;
            justify-content: center;
            align-items: center;
            background: linear-gradient(135deg, #2a0e1f 0%, #4a1942 50%, #6b2463 100%);
            transition: background 0.8s ease;
        }

        body.blink {
            background: linear-gradient(135deg, #ff6b9d 0%, #c44569 50%, #8b1a5e 100%);
        }

        .container {
            text-align: center;
            padding: 3rem;
            background: rgba(255, 255, 255, 0.05);
            backdrop-filter: blur(10px);
            border-radius: 20px;
            border: 1px solid rgba(255, 255, 255, 0.1);
            box-shadow: 0 8px 32px rgba(0, 0, 0, 0.3);
        }

        h1 {
            color: #fff;
            font-size: 2.5rem;
            margin-bottom: 1.5rem;
            font-weight: 300;
            letter-spacing: 2px;
        }

        .controls {
            display: flex;
            gap: 1rem;
            justify-content: center;
            flex-wrap: wrap;
        }

        button {
            padding: 1rem 2rem;
            font-size: 1rem;
            border: none;
            border-radius: 10px;
            cursor: pointer;
            background: linear-gradient(135deg, #ff6b9d, #c44569);
            color: white;
            font-weight: 500;
            transition: transform 0.2s, box-shadow 0.2s;
            box-shadow: 0 4px 15px rgba(255, 107, 157, 0.3);
        }

        button:hover {
            transform: translateY(-2px);
            box-shadow: 0 6px 20px rgba(255, 107, 157, 0.5);
        }

        button:active {
            transform: translateY(0);
        }

        button.stop {
            background: linear-gradient(135deg, #666, #444);
            box-shadow: 0 4px 15px rgba(0, 0, 0, 0.3);
        }

        button.stop:hover {
            box-shadow: 0 6px 20px rgba(0, 0, 0, 0.5);
        }

        .info {
            margin-top: 2rem;
            color: rgba(255, 255, 255, 0.7);
            font-size: 0.9rem;
        }

       let blinkInterval = null;
        const body = document.body;
        const startBtn = document.getElementById('startBtn');
        const stopBtn = document.getElementById('stopBtn');

        startBtn.addEventListener('click', () => {
            if (!blinkInterval) {
                blinkInterval = setInterval(() => {
                    body.classList.toggle('blink');
                }, 800);
            }
        });

        stopBtn.addEventListener('click', () => {
            if (blinkInterval) {
                clearInterval(blinkInterval);
                blinkInterval = null;
                body.classList.remove('blink');
            }
        });