/* CSS Variables based on user's design description */
        :root {
            --background-black: #0A0A0C;
            --panel-background: #1C1C1E;
            --text-color-primary: #FFFFFF;
            --text-color-secondary: #AEAEB2;
            --gradient-purple: #5856d6;
            --gradient-blue: #007AFF;
            --button-primary: #007AFF;
            --button-secondary: #3A3A3C;
            --red-destructive: #FF3B30; /* A vibrant red for delete actions */

            /* Card Colors - Vibrant but balanced */
            --card-color-1: #FFD700; /* Gold */
            --card-color-2: #FF6347; /* Tomato */
            --card-color-3: #4682B4; /* SteelBlue */
            --card-color-4: #9370DB; /* MediumPurple */
            --card-color-5: #32CD32; /* LimeGreen */
            --card-color-6: #FF8C00; /* DarkOrange */
            --card-color-7: #20B2AA; /* LightSeaGreen */

            --border-radius-sm: 8px;
            --border-radius-md: 12px;
            --border-radius-lg: 16px;
        }

        body {
            font-family: 'Inter', sans-serif;
            background-color: var(--background-black);
            color: var(--text-color-primary);
            min-height: 100vh;
            display: flex;
            justify-content: center;
            align-items: flex-start; /* Align to top for better content flow */
            padding: 2rem 1rem; /* Add padding for overall content */
            position: relative;
            overflow-x: hidden; /* Prevent horizontal scrolling */
        }

        /* Radial Gradient Background */
        body::before {
            content: '';
            position: absolute;
            top: 0;
            left: 0;
            right: 0;
            bottom: 0;
            background: radial-gradient(circle at center, var(--gradient-purple), var(--gradient-blue)) 0.15;
            opacity: 0.15;
            z-index: -1;
        }

        /* Main container styling */
        .container {
            width: 100%;
            max-width: 1200px;
            display: flex;
            flex-direction: column;
            gap: 2rem;
        }

        /* Panel styling for input and flashcards */
        .panel {
            background-color: var(--panel-background);
            border-radius: var(--border-radius-lg); /* Slightly larger radius for panels */
            padding: 1.5rem;
            box-shadow: 0 6px 20px rgba(0, 0, 0, 0.4), inset 0 1px 0 rgba(255, 255, 255, 0.05); /* Enhanced shadow and subtle inner highlight */
            transition: all 0.3s ease-in-out;
        }

        /* Input field styling */
        input[type="text"], textarea {
            width: 100%;
            padding: 0.75rem 1rem;
            border-radius: var(--border-radius-sm);
            background-color: var(--button-secondary); /* Use secondary button color for input background */
            border: 1px solid rgba(255, 255, 255, 0.1);
            color: var(--text-color-primary);
            font-size: 1rem;
            transition: border-color 0.2s ease, box-shadow 0.2s ease;
            box-shadow: inset 0 1px 3px rgba(0, 0, 0, 0.3); /* Subtle inner shadow for depth */
        }

        input[type="text"]:focus, textarea:focus {
            outline: none;
            border-color: var(--button-primary);
            box-shadow: 0 0 0 3px rgba(0, 122, 255, 0.4), inset 0 1px 3px rgba(0, 0, 0, 0.3); /* Glow effect on focus */
        }

        /* Placeholder color */
        input::placeholder, textarea::placeholder {
            color: var(--text-color-secondary);
            opacity: 0.7;
        }

        /* Button styling */
        .btn {
            padding: 0.75rem 1.5rem;
            border-radius: var(--border-radius-sm);
            font-weight: 600;
            cursor: pointer;
            transition: background-color 0.2s ease, transform 0.2s ease, box-shadow 0.2s ease;
            display: flex;
            align-items: center;
            justify-content: center;
            gap: 0.5rem;
            box-shadow: 0 2px 5px rgba(0, 0, 0, 0.3); /* Subtle button shadow */
        }

        .btn-primary {
            background-color: var(--button-primary);
            color: var(--text-color-primary);
            background-image: linear-gradient(145deg, rgba(255,255,255,0.1) 0%, rgba(255,255,255,0) 100%); /* Subtle gradient */
        }

        .btn-primary:hover {
            background-color: #0066CC; /* Slightly darker blue on hover */
            transform: translateY(-2px);
            box-shadow: 0 4px 10px rgba(0, 0, 0, 0.4);
        }

        .btn-secondary {
            background-color: var(--button-secondary);
            color: var(--text-color-primary);
            background-image: linear-gradient(145deg, rgba(255,255,255,0.05) 0%, rgba(255,255,255,0) 100%); /* Subtle gradient */
        }

        .btn-secondary:hover {
            background-color: #4A4A4C; /* Slightly lighter grey on hover */
            transform: translateY(-2px);
            box-shadow: 0 4px 10px rgba(0, 0, 0, 0.4);
        }

        .btn-destructive {
            background-color: var(--red-destructive);
            color: var(--text-color-primary);
            background-image: linear-gradient(145deg, rgba(255,255,255,0.1) 0%, rgba(255,255,255,0) 100%);
        }

        .btn-destructive:hover {
            background-color: #CC0000;
            transform: translateY(-2px);
            box-shadow: 0 4px 10px rgba(0, 0, 0, 0.4);
        }

        /* Flashcard specific styling */
        .flashcard-grid {
            display: grid;
            grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
            gap: 1.5rem;
            margin-top: 1.5rem;
        }

        .flashcard-container {
            perspective: 1000px; /* For 3D flip effect */
            height: 200px; /* Fixed height for consistency */
            position: relative; /* Needed for delete button positioning */
            opacity: 0; /* Initial state for fade-in animation */
            transform: translateY(20px); /* Initial state for slide-up animation */
            animation: fadeInSlideUp 0.5s forwards ease-out; /* Animation on card creation */
        }

        .flashcard-container.removing {
            animation: fadeOutSlideDown 0.5s forwards ease-in; /* Animation on card removal */
        }

        @keyframes fadeInSlideUp {
            to {
                opacity: 1;
                transform: translateY(0);
            }
        }

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


        .flashcard {
            width: 100%;
            height: 100%;
            position: relative;
            transform-style: preserve-3d;
            transition: transform 0.6s cubic-bezier(0.68, -0.55, 0.265, 1.55); /* More bouncy flip animation */
            border-radius: var(--border-radius-md);
            box-shadow: 0 4px 15px rgba(0, 0, 0, 0.5), inset 0 1px 0 rgba(255, 255, 255, 0.1); /* Enhanced shadow and inner highlight */
            cursor: pointer;
            display: flex;
            justify-content: center;
            align-items: center;
            text-align: center;
            padding: 1rem;
            color: var(--text-color-primary); /* Ensure text is visible on colored cards */
            transition: transform 0.3s ease, box-shadow 0.3s ease; /* For hover effect */
        }

        .flashcard:hover {
            transform: translateY(-5px); /* Slight lift on hover */
            box-shadow: 0 8px 25px rgba(0, 0, 0, 0.6), inset 0 1px 0 rgba(255, 255, 255, 0.15);
        }

        .flashcard.flipped {
            transform: rotateY(180deg);
        }

        .flashcard-front, .flashcard-back {
            position: absolute;
            width: 100%;
            height: 100%;
            backface-visibility: hidden; /* Hide the back side when not flipped */
            display: flex;
            justify-content: center;
            align-items: center;
            border-radius: var(--border-radius-md);
            padding: 1rem;
            word-break: break-word; /* Allow long words to break */
            overflow: auto; /* Add scroll if content overflows */
            -webkit-overflow-scrolling: touch; /* Smooth scrolling on iOS */
            box-shadow: inset 0 0 10px rgba(0, 0, 0, 0.2); /* Inner shadow for depth */
        }

        .flashcard-front {
            background-color: var(--card-color-1); /* Default, will be overridden by JS */
        }

        .flashcard-back {
            background-color: var(--card-color-1); /* Default, will be overridden by JS */
            transform: rotateY(180deg);
        }

        /* Specific card color classes */
        .card-color-1 { background-color: var(--card-color-1); }
        .card-color-2 { background-color: var(--card-color-2); }
        .card-color-3 { background-color: var(--card-color-3); }
        .card-color-4 { background-color: var(--card-color-4); }
        .card-color-5 { background-color: var(--card-color-5); }
        .card-color-6 { background-color: var(--card-color-6); }
        .card-color-7 { background-color: var(--card-color-7); }

        /* Delete button on flashcard */
        .delete-card-btn {
            position: absolute;
            top: 0.5rem;
            right: 0.5rem;
            background-color: rgba(0, 0, 0, 0.6); /* Semi-transparent dark background */
            color: var(--red-destructive);
            border-radius: 50%;
            width: 32px;
            height: 32px;
            display: flex;
            justify-content: center;
            align-items: center;
            cursor: pointer;
            z-index: 10; /* Ensure it's above the card content */
            opacity: 0; /* Hidden by default */
            transition: opacity 0.2s ease, transform 0.2s ease;
            box-shadow: 0 2px 5px rgba(0, 0, 0, 0.3);
        }

        .flashcard-container:hover .delete-card-btn {
            opacity: 1; /* Show on container hover */
            transform: scale(1.1);
        }

        .delete-card-btn:hover {
            background-color: rgba(0, 0, 0, 0.8);
            transform: scale(1.2);
        }

        /* Custom Modal Styling */
        .modal-overlay {
            position: fixed;
            top: 0;
            left: 0;
            width: 100%;
            height: 100%;
            background-color: rgba(0, 0, 0, 0.7); /* Dark overlay */
            display: flex;
            justify-content: center;
            align-items: center;
            z-index: 1000;
            opacity: 0;
            visibility: hidden;
            transition: opacity 0.3s ease, visibility 0.3s ease;
        }

        .modal-overlay.visible {
            opacity: 1;
            visibility: visible;
        }

        .modal-content {
            background-color: var(--panel-background);
            border-radius: var(--border-radius-lg);
            padding: 2rem;
            max-width: 400px;
            width: 90%;
            text-align: center;
            box-shadow: 0 10px 30px rgba(0, 0, 0, 0.6), inset 0 1px 0 rgba(255, 255, 255, 0.08);
            transform: scale(0.9);
            opacity: 0;
            transition: transform 0.3s cubic-bezier(0.68, -0.55, 0.265, 1.55), opacity 0.3s ease;
            backdrop-filter: blur(10px); /* Frosted glass effect */
            -webkit-backdrop-filter: blur(10px);
        }

        .modal-overlay.visible .modal-content {
            transform: scale(1);
            opacity: 1;
        }

        .modal-content h3 {
            font-size: 1.5rem;
            font-weight: 600;
            margin-bottom: 1rem;
            color: var(--text-color-primary);
        }

        .modal-content p {
            font-size: 1rem;
            color: var(--text-color-secondary);
            margin-bottom: 1.5rem;
        }

        .modal-buttons {
            display: flex;
            justify-content: center;
            gap: 1rem;
        }

        /* Animation for "No flashcards" message */
        #no-flashcards-message {
            animation: pulse 2s infinite ease-in-out;
        }

        @keyframes pulse {
            0% { opacity: 0.7; }
            50% { opacity: 1; }
            100% { opacity: 0.7; }
        }


        /* Responsive adjustments */
        @media (max-width: 768px) {
            .container {
                padding: 1rem;
            }
            .flashcard-grid {
                grid-template-columns: 1fr; /* Stack cards on small screens */
            }
            .panel {
                padding: 1rem;
            }
            .flashcard-container {
                height: 180px; /* Slightly smaller height on mobile */
            }
            .modal-content {
                padding: 1.5rem;
            }
        }