:root {
    --bg-primary: #0f172a;
    --bg-secondary: #1e293b;
    --bg-tertiary: #334155;
    --accent-primary: #3b82f6;
    --accent-secondary: #8b5cf6;
    --accent-success: #10b981;
    --accent-error: #ef4444;
    --accent-warning: #f59e0b;
    --text-primary: #f1f5f9;
    --text-secondary: #94a3b8;
    --border-color: #475569;
    --glass-bg: rgba(30, 41, 59, 0.7);
    --glass-border: rgba(148, 163, 184, 0.1);
}

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

body {
    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
    background: linear-gradient(135deg, #0f172a 0%, #1e293b 100%);
    color: var(--text-primary);
    min-height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    padding: 20px;
}

.container {
    width: 100%;
    max-width: 600px;
}

header {
    background: var(--glass-bg);
    backdrop-filter: blur(10px);
    border: 1px solid var(--glass-border);
    border-radius: 16px;
    padding: 24px;
    margin-bottom: 24px;
}

.header-content {
    display: flex;
    justify-content: space-between;
    align-items: center;
}

h1 {
    font-size: 32px;
    font-weight: 700;
    display: flex;
    align-items: center;
    gap: 12px;
    color: var(--text-primary);
}

h1 i {
    color: var(--accent-primary);
}

.timer {
    display: flex;
    align-items: center;
    gap: 8px;
    font-size: 24px;
    font-weight: 600;
    color: var(--accent-secondary);
}

.timer i {
    width: 24px;
    height: 24px;
}

main {
    background: var(--glass-bg);
    backdrop-filter: blur(10px);
    border: 1px solid var(--glass-border);
    border-radius: 16px;
    padding: 24px;
}

.game-info {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 16px;
    margin-bottom: 24px;
}

.info-card {
    background: var(--bg-secondary);
    border: 1px solid var(--border-color);
    border-radius: 12px;
    padding: 16px;
    display: flex;
    align-items: center;
    gap: 12px;
}

.info-card i {
    width: 24px;
    height: 24px;
    color: var(--accent-primary);
}

.info-card .label {
    display: block;
    font-size: 12px;
    color: var(--text-secondary);
    text-transform: uppercase;
    letter-spacing: 0.5px;
}

.info-card .value {
    display: block;
    font-size: 18px;
    font-weight: 600;
    color: var(--text-primary);
}

.sudoku-board {
    display: grid;
    grid-template-columns: repeat(9, 1fr);
    gap: 0;
    background: var(--bg-primary);
    border: 3px solid var(--accent-primary);
    border-radius: 8px;
    overflow: hidden;
    margin-bottom: 24px;
    aspect-ratio: 1;
}

.cell {
    aspect-ratio: 1;
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 20px;
    font-weight: 600;
    background: var(--bg-secondary);
    border: 1px solid var(--border-color);
    cursor: pointer;
    transition: all 0.2s ease;
    position: relative;
}

.cell:hover:not(.fixed):not(.completed) {
    background: var(--bg-tertiary);
    transform: scale(1.05);
}

.cell.selected {
    background: var(--accent-primary);
    color: var(--bg-primary);
    box-shadow: 0 0 20px rgba(59, 130, 246, 0.5);
}

.cell.fixed {
    color: var(--text-primary);
    font-weight: 700;
    cursor: default;
}

.cell.user-input {
    color: var(--accent-secondary);
}

.cell.error {
    background: var(--accent-error);
    color: var(--text-primary);
    animation: shake 0.3s ease;
}

.cell.hint {
    background: var(--accent-success);
    color: var(--bg-primary);
    animation: pulse 0.5s ease;
}

.cell.completed {
    background: var(--accent-success);
    color: var(--bg-primary);
}

.cell.same-number {
    background: rgba(59, 130, 246, 0.2);
}

.cell.notes {
    font-size: 10px;
    padding: 2px;
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    grid-template-rows: repeat(3, 1fr);
    gap: 1px;
}

.cell.notes span {
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 8px;
    color: var(--text-secondary);
}

/* 3x3 box borders */
.cell:nth-child(3n) {
    border-right: 2px solid var(--accent-primary);
}

.cell:nth-child(n+19):nth-child(-n+27),
.cell:nth-child(n+46):nth-child(-n+54) {
    border-bottom: 2px solid var(--accent-primary);
}

.number-pad {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    gap: 12px;
    margin-bottom: 24px;
}

.number-btn,
.action-btn {
    padding: 16px;
    font-size: 18px;
    font-weight: 600;
    background: var(--bg-secondary);
    border: 2px solid var(--border-color);
    border-radius: 12px;
    color: var(--text-primary);
    cursor: pointer;
    transition: all 0.2s ease;
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 8px;
}

.number-btn:hover,
.action-btn:hover {
    background: var(--bg-tertiary);
    border-color: var(--accent-primary);
    transform: translateY(-2px);
}

.number-btn:active,
.action-btn:active {
    transform: translateY(0);
}

.action-btn i {
    width: 18px;
    height: 18px;
}

.action-btn.active {
    background: var(--accent-primary);
    border-color: var(--accent-primary);
    color: var(--bg-primary);
}

.controls {
    display: flex;
    flex-direction: column;
    gap: 16px;
}

.difficulty-selector {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    gap: 12px;
}

.difficulty-btn {
    padding: 12px;
    font-size: 14px;
    font-weight: 600;
    background: var(--bg-secondary);
    border: 2px solid var(--border-color);
    border-radius: 8px;
    color: var(--text-primary);
    cursor: pointer;
    transition: all 0.2s ease;
    text-transform: uppercase;
    letter-spacing: 0.5px;
}

.difficulty-btn:hover {
    border-color: var(--accent-primary);
}

.difficulty-btn.active {
    background: var(--accent-primary);
    border-color: var(--accent-primary);
    color: var(--bg-primary);
}

.new-game-btn {
    padding: 16px;
    font-size: 16px;
    font-weight: 600;
    background: linear-gradient(135deg, var(--accent-primary) 0%, var(--accent-secondary) 100%);
    border: none;
    border-radius: 12px;
    color: var(--text-primary);
    cursor: pointer;
    transition: all 0.2s ease;
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 8px;
}

.new-game-btn:hover {
    transform: translateY(-2px);
    box-shadow: 0 8px 20px rgba(59, 130, 246, 0.3);
}

.new-game-btn i {
    width: 20px;
    height: 20px;
}

.modal {
    display: none;
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: rgba(15, 23, 42, 0.9);
    backdrop-filter: blur(10px);
    z-index: 1000;
    align-items: center;
    justify-content: center;
}

.modal.show {
    display: flex;
    animation: fadeIn 0.3s ease;
}

.modal-content {
    background: var(--glass-bg);
    backdrop-filter: blur(20px);
    border: 2px solid var(--glass-border);
    border-radius: 24px;
    padding: 48px;
    text-align: center;
    max-width: 400px;
    animation: slideUp 0.3s ease;
}

.modal-icon {
    width: 80px;
    height: 80px;
    margin: 0 auto 24px;
    background: linear-gradient(135deg, var(--accent-primary) 0%, var(--accent-secondary) 100%);
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
}

.modal-icon i {
    width: 48px;
    height: 48px;
    color: var(--text-primary);
}

.modal-content h2 {
    font-size: 32px;
    margin-bottom: 12px;
    color: var(--text-primary);
}

.modal-content p {
    font-size: 18px;
    color: var(--text-secondary);
    margin-bottom: 24px;
}

.modal-stats {
    display: flex;
    justify-content: center;
    gap: 32px;
    margin-bottom: 32px;
}

.modal-stats .stat {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 8px;
}

.modal-stats .stat i {
    width: 24px;
    height: 24px;
    color: var(--accent-primary);
}

.modal-stats .stat span {
    font-size: 18px;
    font-weight: 600;
    color: var(--text-primary);
}

.modal-btn {
    padding: 16px 32px;
    font-size: 16px;
    font-weight: 600;
    background: linear-gradient(135deg, var(--accent-primary) 0%, var(--accent-secondary) 100%);
    border: none;
    border-radius: 12px;
    color: var(--text-primary);
    cursor: pointer;
    transition: all 0.2s ease;
    display: inline-flex;
    align-items: center;
    gap: 8px;
}

.modal-btn:hover {
    transform: translateY(-2px);
    box-shadow: 0 8px 20px rgba(59, 130, 246, 0.3);
}

.modal-btn i {
    width: 20px;
    height: 20px;
}

@keyframes shake {
    0%, 100% { transform: translateX(0); }
    25% { transform: translateX(-5px); }
    75% { transform: translateX(5px); }
}

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

@keyframes fadeIn {
    from { opacity: 0; }
    to { opacity: 1; }
}

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

@media (max-width: 640px) {
    .container {
        padding: 0;
    }

    header {
        padding: 16px;
        margin-bottom: 16px;
    }

    h1 {
        font-size: 24px;
    }

    .timer {
        font-size: 18px;
    }

    main {
        padding: 16px;
    }

    .cell {
        font-size: 16px;
    }

    .number-pad {
        gap: 8px;
    }

    .number-btn,
    .action-btn {
        padding: 12px;
        font-size: 16px;
    }

    .modal-content {
        padding: 32px 24px;
        margin: 20px;
    }
}