* {
    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: #9ca3af;
    padding: 20px;
}

.container {
    width: 100%;
    max-width: 700px;
}

header {
    text-align: center;
    color: white;
    margin-bottom: 30px;
}

header h1 {
    font-size: 2.5rem;
    margin-bottom: 10px;
    text-shadow: 2px 2px 4px rgba(0, 0, 0, 0.3);
}

.subtitle {
    font-size: 1.1rem;
    opacity: 0.95;
}

.game-container {
    background: rgba(255, 255, 255, 0.95);
    border-radius: 20px;
    padding: 30px;
    box-shadow: 0 10px 40px rgba(0, 0, 0, 0.3);
}

.game-controls {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 20px;
    flex-wrap: nowrap;
    gap: 10px;
}

.action-buttons {
    display: flex;
    gap: 5px;
}

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

.difficulty-label {
    font-weight: 600;
    color: #4b5563;
}

.difficulty-buttons {
    display: flex;
    gap: 8px;
}

.difficulty-btn {
    padding: 8px 16px;
    border: 2px solid #6366f1;
    background: white;
    color: #6366f1;
    border-radius: 8px;
    font-weight: 600;
    cursor: pointer;
    transition: all 0.2s ease;
}

.difficulty-btn:hover {
    background: #eef2ff;
}

.difficulty-btn.active {
    background: #6366f1;
    color: white;
}

.control-button {
    background: linear-gradient(135deg, #6366f1 0%, #8b5cf6 100%);
    color: white;
    border: none;
    padding: 8px 16px;
    border-radius: 10px;
    font-size: 0.9rem;
    font-weight: 600;
    cursor: pointer;
    transition: all 0.2s ease;
    box-shadow: 0 4px 12px rgba(99, 102, 241, 0.3);
    white-space: nowrap;
}

.control-button:hover {
    transform: scale(1.05);
    box-shadow: 0 5px 15px rgba(99, 102, 241, 0.4);
}

.control-button.timer-stopped {
    background: linear-gradient(135deg, #ef4444 0%, #dc2626 100%);
}

.control-button.timer-stopped:hover {
    box-shadow: 0 5px 15px rgba(239, 68, 68, 0.4);
}

.control-button:active {
    transform: scale(0.98);
}

.game-info {
    display: flex;
    justify-content: space-around;
    align-items: center;
    margin-bottom: 20px;
    padding: 15px;
    background: #f3f4f6;
    border-radius: 12px;
    flex-wrap: wrap;
    gap: 10px;
}

.info-item {
    display: flex;
    align-items: center;
    gap: 8px;
}

.info-item .label {
    font-size: 0.95rem;
    font-weight: 600;
    color: #4b5563;
}

.info-item .value {
    font-size: 1.1rem;
    font-weight: 700;
    color: #6366f1;
    min-width: 40px;
    text-align: center;
}

.sudoku-grid {
    display: grid;
    grid-template-columns: repeat(9, 1fr);
    gap: 0;
    width: 100%;
    max-width: 500px;
    margin: 0 auto 20px;
    border: 3px solid #1f2937;
    background: #1f2937;
    aspect-ratio: 1;
}

.cell {
    aspect-ratio: 1;
    background: white;
    border: 1px solid #6b7280;
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 1.5rem;
    font-weight: 600;
    cursor: pointer;
    transition: all 0.2s ease;
    color: #6366f1;
}

.cell:hover:not(.given):not(.correct) {
    background: #eef2ff;
}

.cell.selected {
    background: #dbeafe;
    box-shadow: inset 0 0 0 2px #3b82f6;
}

.cell.given {
    background: #f3f4f6;
    color: #1f2937;
    font-weight: 700;
    cursor: default;
}

.cell.error {
    background: #fee2e2;
    color: #dc2626;
}

.cell.correct {
    background: #d1fae5;
    color: #059669;
}

.cell.hint {
    background: #fef3c7;
    color: #d97706;
    animation: hintPulse 0.5s ease;
}

@keyframes hintPulse {
    0%, 100% { transform: scale(1); }
    50% { transform: scale(1.1); }
}

/* 3x3 box borders */
.cell:nth-child(3n) {
    border-right: 3px solid #6b7280;
}

.cell:nth-child(n+19):nth-child(-n+27),
.cell:nth-child(n+46):nth-child(-n+54) {
    border-bottom: 3px solid #6b7280;
}

.number-pad {
    display: grid;
    grid-template-columns: repeat(10, 1fr);
    gap: 6px;
    max-width: 500px;
    margin: 0 auto 20px;
}

.number-btn {
    padding: 12px 6px;
    background: white;
    border: 2px solid #6366f1;
    color: #6366f1;
    border-radius: 8px;
    font-size: 1rem;
    font-weight: 700;
    cursor: pointer;
    transition: all 0.2s ease;
}

.number-btn:hover {
    background: #6366f1;
    color: white;
    transform: scale(1.05);
}

.number-btn:active {
    transform: scale(0.95);
}

.number-btn.completed {
    background: #6366f1;
    color: white;
    opacity: 0.6;
    cursor: not-allowed;
}

.number-btn.completed:hover {
    transform: none;
    background: #6366f1;
}

.erase-btn {
    background: #ef4444;
    border-color: #ef4444;
    color: white;
}

.erase-btn:hover {
    background: #dc2626;
    border-color: #dc2626;
}

.win-message {
    text-align: center;
    padding: 30px;
    background: #e5e7eb;
    border-radius: 15px;
    margin-top: 20px;
}

.win-message.hidden {
    display: none;
}

.win-message h2 {
    font-size: 2rem;
    margin-bottom: 10px;
    color: #1f2937;
}

.win-message p {
    font-size: 1.1rem;
    margin-bottom: 15px;
    color: #4b5563;
}

.final-stats {
    font-size: 1rem;
    font-weight: 600;
    color: #6b7280;
    margin-bottom: 20px;
}

.help-button-container {
    position: absolute;
    top: 20px;
    right: 20px;
}

.help-button {
    display: inline-block;
    color: white;
    background: rgba(255, 255, 255, 0.2);
    backdrop-filter: blur(10px);
    border: none;
    padding: 12px 24px;
    border-radius: 20px;
    font-weight: 600;
    cursor: pointer;
    transition: all 0.2s ease;
}

.help-button:hover {
    background: rgba(255, 255, 255, 0.3);
}

.modal {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    z-index: 1000;
    display: flex;
    align-items: center;
    justify-content: center;
}

.modal.hidden {
    display: none;
}

.modal-overlay {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: rgba(0, 0, 0, 0.7);
    backdrop-filter: blur(5px);
}

.modal-content {
    position: relative;
    background: white;
    border-radius: 20px;
    padding: 40px;
    max-width: 600px;
    width: 90%;
    max-height: 80vh;
    overflow-y: auto;
    box-shadow: 0 20px 60px rgba(0, 0, 0, 0.5);
    z-index: 1001;
}

.modal-close {
    position: absolute;
    top: 15px;
    right: 15px;
    background: transparent;
    border: none;
    font-size: 1.5rem;
    color: #6b7280;
    cursor: pointer;
    width: 35px;
    height: 35px;
    display: flex;
    align-items: center;
    justify-content: center;
    border-radius: 50%;
    transition: all 0.2s ease;
}

.modal-close:hover {
    background: #f3f4f6;
    color: #1f2937;
}

.modal-content h2 {
    color: #1f2937;
    font-size: 1.8rem;
    margin-bottom: 20px;
}

.modal-content p {
    color: #4b5563;
    line-height: 1.6;
    margin-bottom: 15px;
}

.modal-content ol,
.modal-content ul {
    margin-left: 20px;
    margin-top: 10px;
    margin-bottom: 15px;
    color: #4b5563;
    line-height: 1.8;
}

.modal-content strong {
    color: #1f2937;
}

.back-button-container {
    position: absolute;
    top: 20px;
    left: 20px;
}

.back-button {
    display: inline-block;
    color: white;
    text-decoration: none;
    padding: 12px 24px;
    border-radius: 20px;
    background: rgba(255, 255, 255, 0.2);
    backdrop-filter: blur(10px);
    font-weight: 600;
    transition: all 0.2s ease;
}

.back-button:hover {
    background: rgba(255, 255, 255, 0.3);
    transform: translateX(-5px);
}

@media (max-width: 768px) {
    header {
        margin-bottom: 60px;
    }

    header h1 {
        display: none;
    }

    .subtitle {
        display: none;
    }

    .game-container {
        padding: 20px 15px;
    }

    .cell {
        font-size: 1.2rem;
    }

    .number-pad {
        grid-template-columns: repeat(10, 1fr);
        gap: 4px;
    }

    .number-btn {
        padding: 10px 4px;
        font-size: 0.9rem;
    }

    .game-controls {
        flex-direction: column;
        align-items: stretch;
    }

    .difficulty-selector {
        flex-direction: column;
        align-items: stretch;
    }

    .difficulty-buttons {
        justify-content: stretch;
    }

    .difficulty-btn {
        flex: 1;
    }

    /* Mobile-friendly: icon-only buttons */
    .back-button {
        font-size: 0;
        padding: 8px;
        min-width: 36px;
        min-height: 36px;
        border-radius: 50%;
        width: 36px;
        height: 36px;
        display: flex;
        align-items: center;
        justify-content: center;
    }

    .back-button::before {
        content: '←';
        font-size: 1.2rem;
    }

    .help-button {
        font-size: 0;
        padding: 8px;
        min-width: 36px;
        min-height: 36px;
    }

    .help-button::before {
        content: '📖';
        font-size: 1.2rem;
    }
}