:root {
            --primary: #4361ee;
            --primary-dark: #3a56d4;
            --primary-light: rgba(67, 97, 238, 0.1);
            --secondary: #7209b7;
            --success: #4cc9f0;
            --success-dark: #3ab8dd;
            --warning: #f8961e;
            --danger: #f72585;
            --light: #f8f9fa;
            --dark: #212529;
            --gray: #6c757d;
            --gray-light: #e9ecef;
            --border-radius: 12px;
            --box-shadow: 0 6px 20px rgba(0, 0, 0, 0.08);
            --transition: all 0.3s ease;
        }

        * {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
            font-family: 'Segoe UI', system-ui, -apple-system, sans-serif;
        }

        body {
            background: linear-gradient(135deg, #f8f9ff 0%, #f0f2ff 100%);
            color: var(--dark);
            min-height: 100vh;
            padding: 20px;
            line-height: 1.6;
        }

        .container {
            max-width: 1400px;
            margin: 0 auto;
        }

        header {
            text-align: center;
            margin-bottom: 40px;
            padding: 20px;
        }

        h1 {
            color: var(--primary);
            margin-bottom: 15px;
            font-size: 2.8rem;
            font-weight: 700;
            display: flex;
            align-items: center;
            justify-content: center;
            gap: 15px;
        }

        .subtitle {
            color: var(--gray);
            font-size: 1.2rem;
            max-width: 800px;
            margin: 0 auto;
        }

        .card {
            background-color: white;
            border-radius: var(--border-radius);
            box-shadow: var(--box-shadow);
            padding: 35px;
            margin-bottom: 30px;
            transition: var(--transition);
        }

        .card:hover {
            box-shadow: 0 10px 25px rgba(0, 0, 0, 0.12);
        }

        .upload-section {
            text-align: center;
            border: 3px dashed var(--primary);
            padding: 40px 30px;
            border-radius: var(--border-radius);
            background-color: rgba(255, 255, 255, 0.7);
            transition: var(--transition);
            position: relative;
            overflow: hidden;
        }

        .upload-section::before {
            content: '';
            position: absolute;
            top: 0;
            left: 0;
            right: 0;
            height: 4px;
            background: linear-gradient(90deg, var(--primary), var(--secondary));
        }

        .upload-section.dragover {
            background-color: rgba(67, 97, 238, 0.05);
            border-color: var(--secondary);
            transform: translateY(-5px);
        }

        .upload-icon {
            font-size: 65px;
            color: var(--primary);
            margin-bottom: 20px;
        }

        .upload-text {
            font-size: 1.4rem;
            margin-bottom: 10px;
            font-weight: 600;
            color: var(--dark);
        }

        .file-input {
            display: none;
        }

        .file-label {
            background: linear-gradient(135deg, var(--primary), var(--secondary));
            color: white;
            padding: 14px 35px;
            border-radius: 50px;
            cursor: pointer;
            display: inline-flex;
            align-items: center;
            justify-content: center;
            gap: 10px;
            margin: 20px 0;
            font-weight: 600;
            font-size: 1.1rem;
            transition: var(--transition);
            box-shadow: 0 4px 12px rgba(67, 97, 238, 0.25);
        }

        .file-label:hover {
            transform: translateY(-3px);
            box-shadow: 0 6px 18px rgba(67, 97, 238, 0.35);
        }

        .file-list {
            margin-top: 30px;
            display: none;
        }

        .file-list.show {
            display: block;
            animation: fadeIn 0.5s ease;
        }

        @keyframes fadeIn {
            from { opacity: 0; transform: translateY(10px); }
            to { opacity: 1; transform: translateY(0); }
        }

        .list-header {
            display: flex;
            justify-content: space-between;
            align-items: center;
            margin-bottom: 20px;
            padding-bottom: 15px;
            border-bottom: 2px solid var(--gray-light);
        }

        .list-title {
            font-size: 1.3rem;
            color: var(--primary);
            font-weight: 700;
            display: flex;
            align-items: center;
            gap: 10px;
        }

        .list-count {
            background-color: var(--primary);
            color: white;
            padding: 5px 12px;
            border-radius: 20px;
            font-size: 0.9rem;
            font-weight: 600;
        }

        .files-container {
            max-height: 400px;
            overflow-y: auto;
            border-radius: var(--border-radius);
            border: 1px solid var(--gray-light);
        }

        .file-item {
            display: flex;
            align-items: center;
            padding: 18px 20px;
            border-bottom: 1px solid var(--gray-light);
            background-color: white;
            transition: var(--transition);
        }

        .file-item:hover {
            background-color: var(--primary-light);
        }

        .file-item:last-child {
            border-bottom: none;
        }

        .file-icon {
            font-size: 24px;
            color: var(--primary);
            width: 50px;
            text-align: center;
        }

        .file-details {
            flex: 1;
            min-width: 0;
        }

        .file-name {
            font-weight: 600;
            color: var(--dark);
            margin-bottom: 5px;
            word-break: break-all;
        }

        .file-meta {
            display: flex;
            gap: 15px;
            color: var(--gray);
            font-size: 0.9rem;
        }

        .file-actions {
            display: flex;
            gap: 10px;
            align-items: center;
        }

        .file-order {
            display: flex;
            flex-direction: column;
            gap: 5px;
            margin-right: 15px;
        }

        .order-btn {
            background: none;
            border: none;
            color: var(--gray);
            cursor: pointer;
            font-size: 1.2rem;
            padding: 2px 8px;
            border-radius: 4px;
            transition: var(--transition);
        }

        .order-btn:hover {
            background-color: var(--primary-light);
            color: var(--primary);
        }

        .remove-btn {
            background: none;
            border: none;
            color: var(--danger);
            cursor: pointer;
            font-size: 1.3rem;
            padding: 8px;
            border-radius: 50%;
            transition: var(--transition);
        }

        .remove-btn:hover {
            background-color: rgba(247, 37, 133, 0.1);
        }

        .size-warning {
            color: var(--danger);
            font-weight: 600;
            margin-top: 20px;
            display: none;
            align-items: center;
            gap: 10px;
            background-color: rgba(247, 37, 133, 0.1);
            padding: 15px;
            border-radius: var(--border-radius);
            border-left: 4px solid var(--danger);
        }

        .size-warning.show {
            display: flex;
        }

        .merge-options {
            margin-top: 30px;
            display: grid;
            grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
            gap: 25px;
        }

        .section-title {
            font-size: 1.3rem;
            margin-bottom: 20px;
            color: var(--primary);
            display: flex;
            align-items: center;
            gap: 10px;
        }

        .option-group {
            background-color: var(--light);
            padding: 25px;
            border-radius: var(--border-radius);
        }

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

        .input-label {
            display: block;
            margin-bottom: 10px;
            font-weight: 600;
            color: var(--dark);
        }

        .custom-input, .custom-select {
            width: 100%;
            padding: 14px;
            border: 2px solid var(--gray-light);
            border-radius: var(--border-radius);
            font-size: 1rem;
            transition: var(--transition);
            background-color: white;
        }

        .custom-input:focus, .custom-select:focus {
            outline: none;
            border-color: var(--primary);
            box-shadow: 0 0 0 3px rgba(67, 97, 238, 0.2);
        }

        .checkbox-group {
            display: flex;
            align-items: center;
            gap: 10px;
            margin-bottom: 15px;
        }

        .checkbox-input {
            width: 18px;
            height: 18px;
            accent-color: var(--primary);
        }

        .action-section {
            display: flex;
            justify-content: center;
            gap: 25px;
            margin-top: 40px;
            flex-wrap: wrap;
        }

        .btn {
            padding: 16px 45px;
            border: none;
            border-radius: 50px;
            font-size: 1.1rem;
            font-weight: 600;
            cursor: pointer;
            transition: var(--transition);
            display: flex;
            align-items: center;
            justify-content: center;
            gap: 12px;
            min-width: 200px;
        }

        .btn-primary {
            background: linear-gradient(135deg, var(--primary), var(--secondary));
            color: white;
            box-shadow: 0 5px 15px rgba(67, 97, 238, 0.3);
        }

        .btn-primary:hover:not(:disabled) {
            transform: translateY(-3px);
            box-shadow: 0 8px 20px rgba(67, 97, 238, 0.4);
        }

        .btn-secondary {
            background-color: white;
            color: var(--dark);
            border: 2px solid var(--gray-light);
        }

        .btn-secondary:hover {
            background-color: var(--light);
            border-color: var(--primary);
            color: var(--primary);
        }

        .btn:disabled {
            opacity: 0.6;
            cursor: not-allowed;
            transform: none !important;
            box-shadow: none !important;
        }

        .progress-section {
            margin-top: 35px;
            display: none;
        }

        .progress-section.show {
            display: block;
            animation: fadeIn 0.5s ease;
        }

        .progress-header {
            display: flex;
            justify-content: space-between;
            align-items: center;
            margin-bottom: 15px;
        }

        .progress-bar {
            height: 22px;
            background-color: var(--gray-light);
            border-radius: 11px;
            overflow: hidden;
            margin-bottom: 10px;
            box-shadow: inset 0 2px 5px rgba(0, 0, 0, 0.1);
        }

        .progress-fill {
            height: 100%;
            background: linear-gradient(90deg, var(--primary), var(--success));
            width: 0%;
            transition: width 0.5s ease;
            border-radius: 11px;
        }

        .progress-text {
            text-align: center;
            font-weight: 600;
            color: var(--primary);
            font-size: 1.1rem;
        }

        .time-estimate {
            color: var(--gray);
            font-size: 0.9rem;
        }

        .processing-details {
            margin-top: 25px;
            padding: 20px;
            background-color: var(--primary-light);
            border-radius: var(--border-radius);
            display: none;
        }

        .processing-details.show {
            display: block;
        }

        .detail-item {
            display: flex;
            justify-content: space-between;
            padding: 10px 0;
            border-bottom: 1px solid rgba(67, 97, 238, 0.1);
        }

        .detail-item:last-child {
            border-bottom: none;
        }

        .results-section {
            margin-top: 35px;
            display: none;
        }

        .results-section.show {
            display: block;
            animation: fadeIn 0.5s ease;
        }

        .results-title {
            font-size: 1.8rem;
            margin-bottom: 25px;
            color: var(--secondary);
            text-align: center;
            display: flex;
            align-items: center;
            justify-content: center;
            gap: 15px;
        }

        .result-card {
            background: linear-gradient(145deg, #ffffff, #f5f7ff);
            border-radius: var(--border-radius);
            padding: 30px;
            box-shadow: var(--box-shadow);
            border-top: 5px solid var(--success);
            transition: var(--transition);
            margin-bottom: 25px;
        }

        .result-card:hover {
            transform: translateY(-5px);
            box-shadow: 0 12px 25px rgba(0, 0, 0, 0.12);
        }

        .result-header {
            display: flex;
            align-items: center;
            justify-content: space-between;
            margin-bottom: 20px;
            flex-wrap: wrap;
            gap: 15px;
        }

        .result-name {
            font-weight: 700;
            word-break: break-all;
            color: var(--dark);
            font-size: 1.4rem;
        }

        .result-meta {
            display: flex;
            flex-wrap: wrap;
            gap: 15px;
            margin-top: 15px;
        }

        .meta-badge {
            background-color: var(--primary-light);
            color: var(--primary);
            padding: 8px 15px;
            border-radius: 20px;
            font-size: 0.9rem;
            font-weight: 600;
            display: inline-flex;
            align-items: center;
            gap: 8px;
        }

        .download-btn {
            background: linear-gradient(135deg, var(--success), var(--primary));
            color: white;
            border: none;
            border-radius: 8px;
            padding: 15px 30px;
            cursor: pointer;
            font-weight: 600;
            font-size: 1.1rem;
            transition: var(--transition);
            display: flex;
            align-items: center;
            justify-content: center;
            gap: 12px;
            box-shadow: 0 4px 10px rgba(76, 201, 240, 0.3);
            margin-top: 20px;
        }

        .download-btn:hover {
            transform: translateY(-3px);
            box-shadow: 0 6px 15px rgba(76, 201, 240, 0.4);
        }

        .security-section {
            background: linear-gradient(135deg, rgba(67, 97, 238, 0.05), rgba(114, 9, 183, 0.05));
            border-radius: var(--border-radius);
            padding: 30px;
            margin-top: 40px;
            border-left: 5px solid var(--primary);
        }

        .security-header {
            display: flex;
            align-items: center;
            gap: 15px;
            margin-bottom: 15px;
        }

        .security-title {
            color: var(--primary);
            font-size: 1.5rem;
            font-weight: 700;
        }

        .security-desc {
            color: var(--gray);
            font-size: 1rem;
            line-height: 1.6;
        }

        .features {
            display: grid;
            grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
            gap: 20px;
            margin-top: 20px;
        }

        .feature {
            display: flex;
            align-items: flex-start;
            gap: 15px;
        }

        .feature-icon {
            color: var(--success);
            font-size: 1.5rem;
            flex-shrink: 0;
        }

        .empty-state {
            text-align: center;
            padding: 40px 20px;
            color: var(--gray);
        }

        .empty-icon {
            font-size: 50px;
            margin-bottom: 15px;
            color: var(--gray-light);
        }

        footer {
            text-align: center;
            margin-top: 50px;
            color: var(--gray);
            font-size: 0.95rem;
            padding: 25px;
            border-top: 1px solid var(--gray-light);
        }

        @media (max-width: 768px) {
            .container {
                padding: 10px;
            }
            
            h1 {
                font-size: 2.2rem;
                flex-direction: column;
                gap: 10px;
            }
            
            .card {
                padding: 25px 20px;
            }
            
            .upload-section {
                padding: 30px 20px;
            }
            
            .action-section {
                flex-direction: column;
                align-items: center;
            }
            
            .btn {
                width: 100%;
                max-width: 350px;
            }
            
            .merge-options {
                grid-template-columns: 1fr;
            }
            
            .file-item {
                flex-wrap: wrap;
            }
            
            .file-actions {
                margin-top: 10px;
                width: 100%;
                justify-content: flex-end;
            }
        }

        .total-size {
            margin-top: 15px;
            padding: 12px;
            background-color: rgba(67, 97, 238, 0.05);
            border-radius: var(--border-radius);
            font-weight: 600;
            color: var(--primary);
            display: flex;
            justify-content: space-between;
        }