* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: 'Poppins', sans-serif;
}

:root {
    --primary: #6366f1;
    --secondary: #8b5cf6;
    --accent: #a78bfa;
    --dark: #1e293b;
    --light: #f8fafc;
    --glass: rgba(255, 255, 255, 0.1);
    --shadow: 0 8px 32px rgba(31, 38, 135, 0.2);
    --border: 1px solid rgba(255, 255, 255, 0.18);
    --transition: all 0.3s ease;
}

body {
    background: linear-gradient(135deg, #0f172a, #1e293b);
    min-height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    padding: 20px;
    color: var(--light);
}

.container {
    max-width: 900px;
    width: 100%;
    background: rgba(30, 41, 59, 0.7);
    backdrop-filter: blur(10px);
    border-radius: 20px;
    overflow: hidden;
    box-shadow: var(--shadow);
    border: var(--border);
    padding: 30px;
}

header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 25px;
}

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

.logo i {
    font-size: 28px;
    color: var(--accent);
    text-shadow: 0 0 15px rgba(167, 139, 250, 0.5);
}

.logo h1 {
    font-size: 28px;
    font-weight: 600;
    background: linear-gradient(to right, var(--primary), var(--accent));
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
}

.game-info {
    display: flex;
    align-items: center;
    gap: 20px;
}

.timer {
    background: var(--glass);
    padding: 8px 15px;
    border-radius: 50px;
    font-size: 18px;
    font-weight: 500;
    display: flex;
    align-items: center;
    gap: 8px;
    border: var(--border);
}

.timer i {
    color: var(--accent);
}

.status {
    background: var(--glass);
    padding: 8px 20px;
    border-radius: 50px;
    font-size: 16px;
    font-weight: 500;
    border: var(--border);
    text-transform: uppercase;
    letter-spacing: 1px;
}

.game-status {
    color: var(--accent);
    font-weight: 600;
}

.game-board {
    display: grid;
    grid-template-columns: 1fr;
    gap: 30px;
}

.sudoku-container {
    display: flex;
    justify-content: center;
}

.sudoku-grid {
    display: grid;
    grid-template-columns: repeat(9, 1fr);
    gap: 0;
    width: min(80vw, 500px);
    height: min(80vw, 500px);
    background: var(--glass);
    border-radius: 8px;
    overflow: hidden;
    border: var(--border);
    box-shadow: 0 10px 20px rgba(0, 0, 0, 0.1);
}

.cell {
    display: flex;
    justify-content: center;
    align-items: center;
    font-size: 24px;
    font-weight: 500;
    background: rgba(30, 41, 59, 0.4);
    border: 1px solid rgba(255, 255, 255, 0.05);
    cursor: pointer;
    transition: var(--transition);
    position: relative;
}

.cell:nth-child(3n) {
    border-right: 2px solid rgba(255, 255, 255, 0.2);
}

.cell:nth-child(9n) {
    border-right: 1px solid rgba(255, 255, 255, 0.05);
}

.cell:nth-child(n+19):nth-child(-n+27),
.cell:nth-child(n+46):nth-child(-n+54) {
    border-bottom: 2px solid rgba(255, 255, 255, 0.2);
}

.cell.prefilled {
    background: rgba(30, 41, 59, 0.6);
    color: #cbd5e1;
    font-weight: 600;
}

.cell.user-input {
    color: var(--accent);
    font-weight: 600;
}

.cell.selected {
    background: rgba(99, 102, 241, 0.2);
    transform: scale(1.05);
    z-index: 2;
    box-shadow: 0 0 15px rgba(99, 102, 241, 0.3);
}

.cell.highlighted {
    background: rgba(139, 92, 246, 0.15);
}

.cell.conflict {
    background: rgba(239, 68, 68, 0.2);
    color: #f87171;
}

.controls {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    gap: 15px;
    margin-top: 20px;
}

.number-pad {
    grid-column: span 2;
    display: grid;
    grid-template-columns: repeat(5, 1fr);
    gap: 10px;
}

.action-buttons {
    display: flex;
    flex-direction: column;
    gap: 10px;
}

.btn {
    padding: 14px;
    border: none;
    border-radius: 12px;
    font-size: 16px;
    font-weight: 500;
    cursor: pointer;
    transition: var(--transition);
    display: flex;
    justify-content: center;
    align-items: center;
    gap: 8px;
    background: linear-gradient(135deg, var(--primary), var(--secondary));
    color: white;
    box-shadow: 0 4px 15px rgba(99, 102, 241, 0.3);
}

.btn:hover {
    transform: translateY(-3px);
    box-shadow: 0 6px 20px rgba(99, 102, 241, 0.4);
}

.btn:active {
    transform: translateY(1px);
}

.btn.secondary {
    background: var(--glass);
    color: var(--light);
    box-shadow: 0 4px 15px rgba(0, 0, 0, 0.1);
}

.btn.secondary:hover {
    background: rgba(255, 255, 255, 0.15);
}

.btn.numpad {
    background: rgba(30, 41, 59, 0.7);
    font-size: 20px;
    font-weight: 600;
}

.btn.numpad:hover {
    background: rgba(99, 102, 241, 0.3);
}

.difficulty {
    display: flex;
    justify-content: center;
    gap: 15px;
    margin-top: 25px;
}

.diff-btn {
    padding: 10px 25px;
    border-radius: 50px;
    background: var(--glass);
    border: var(--border);
    color: var(--light);
    cursor: pointer;
    transition: var(--transition);
}

.diff-btn.active {
    background: linear-gradient(135deg, var(--primary), var(--secondary));
    box-shadow: 0 0 15px rgba(139, 92, 246, 0.4);
}

.hint-indicator {
    display: flex;
    justify-content: center;
    align-items: center;
    gap: 10px;
    margin-top: 20px;
    color: #cbd5e1;
}

.hint-dots {
    display: flex;
    gap: 8px;
}

.hint-dot {
    width: 12px;
    height: 12px;
    border-radius: 50%;
    background: rgba(139, 92, 246, 0.3);
}

.hint-dot.active {
    background: var(--accent);
    box-shadow: 0 0 8px var(--accent);
}

footer {
    margin-top: 30px;
    text-align: center;
    color: #94a3b8;
    font-size: 14px;
}

/* Responsive Design */
@media (max-width: 768px) {
    .controls {
        grid-template-columns: 1fr;
    }
    
    .number-pad {
        grid-column: 1;
        order: 2;
    }
    
    .action-buttons {
        order: 1;
        flex-direction: row;
        flex-wrap: wrap;
    }
    
    .action-buttons .btn {
        flex: 1;
        min-width: 120px;
    }
    
    .difficulty {
        flex-wrap: wrap;
    }
    
    header {
        flex-direction: column;
        gap: 15px;
    }
    
    .game-info {
        width: 100%;
        justify-content: center;
    }
}

@media (max-width: 480px) {
    .btn {
        padding: 12px;
        font-size: 14px;
    }
    
    .timer, .status {
        font-size: 14px;
        padding: 6px 12px;
    }
    
    .cell {
        font-size: 18px;
    }
}

/* Animations */
@keyframes pulse {
    0% { transform: scale(1); }
    50% { transform: scale(1.05); }
    100% { transform: scale(1); }
}

.pulse {
    animation: pulse 0.5s ease;
}

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

.fade-in {
    animation: fadeIn 0.4s ease forwards;
}