:root {
    /* Colors - Light Mode */
    --bg-primary: #f8f9fa;
    --bg-secondary: #ffffff;
    --bg-card: #ffffff;
    --text-primary: #1a1a1a;
    --text-secondary: #6c757d;
    --text-muted: #adb5bd;

    /* Accent Colors */
    --color-primary: #4361ee;
    --color-primary-light: #7692ff;
    --color-primary-dark: #3451d8;
    --color-success: #10b981;
    --color-error: #ef4444;
    --color-warning: #f59e0b;

    /* Board Colors */
    --tile-bg: #ffffff;
    --tile-starter: #f1f3f5;
    --tile-selected: #e3f2fd;
    --tile-hover: #f8f9fa;
    --tile-border: #e9ecef;
    --tile-border-thick: #495057;
    --tile-highlight: #fff3cd;
    --tile-conflict: #fee;

    /* Shadows */
    --shadow-sm: 0 2px 4px rgba(0, 0, 0, 0.05);
    --shadow-md: 0 4px 12px rgba(0, 0, 0, 0.1);
    --shadow-lg: 0 8px 24px rgba(0, 0, 0, 0.15);
    --shadow-glow: 0 0 20px rgba(67, 97, 238, 0.3);

    /* Border Radius */
    --radius-sm: 6px;
    --radius-md: 12px;
    --radius-lg: 16px;
    --radius-xl: 24px;

    /* Transitions */
    --transition-fast: 150ms cubic-bezier(0.4, 0, 0.2, 1);
    --transition-normal: 250ms cubic-bezier(0.4, 0, 0.2, 1);
    --transition-slow: 350ms cubic-bezier(0.4, 0, 0.2, 1);

    /* Spacing */
    --spacing-xs: 4px;
    --spacing-sm: 8px;
    --spacing-md: 16px;
    --spacing-lg: 24px;
    --spacing-xl: 32px;

    /* Typography */
    --font-family: 'Poppins', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
    --font-size-sm: 0.875rem;
    --font-size-base: 1rem;
    --font-size-lg: 1.125rem;
    --font-size-xl: 1.5rem;
    --font-size-2xl: 2rem;
}

/* Dark Mode Colors */
[data-theme="dark"] {
    --bg-primary: #0a0a0a;
    --bg-secondary: #1a1a1a;
    --bg-card: #1f1f1f;
    --text-primary: #f8f9fa;
    --text-secondary: #adb5bd;
    --text-muted: #6c757d;

    --tile-bg: #2a2a2a;
    --tile-starter: #1a1a1a;
    --tile-selected: #1e3a5f;
    --tile-hover: #333333;
    --tile-border: #3a3a3a;
    --tile-border-thick: #666666;
    --tile-highlight: #3d3516;
    --tile-conflict: #3d1616;

    --shadow-sm: 0 2px 4px rgba(0, 0, 0, 0.3);
    --shadow-md: 0 4px 12px rgba(0, 0, 0, 0.4);
    --shadow-lg: 0 8px 24px rgba(0, 0, 0, 0.5);
}

/* ========================================
   BASE STYLES
   ======================================== */
* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    font-family: var(--font-family);
    background: var(--bg-primary);
    color: var(--text-primary);
    min-height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    padding: var(--spacing-lg);
    transition: background-color var(--transition-normal);
}

/* ========================================
   GAME CONTAINER
   ======================================== */
.game-container {
    background: var(--bg-card);
    border-radius: var(--radius-xl);
    box-shadow: var(--shadow-lg);
    padding: var(--spacing-xl);
    max-width: 600px;
    width: 100%;
    transition: all var(--transition-normal);
}

/* ========================================
   HEADER
   ======================================== */
.game-header {
    margin-bottom: var(--spacing-lg);
}

.game-title {
    text-align: center;
    font-size: var(--font-size-2xl);
    font-weight: 700;
    color: var(--color-primary);
    margin-bottom: var(--spacing-lg);
    text-shadow: 0 2px 4px rgba(67, 97, 238, 0.2);
}

.header-stats {
    display: flex;
    justify-content: space-around;
    margin-bottom: var(--spacing-lg);
    gap: var(--spacing-sm);
}

.stat-item {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: var(--spacing-xs);
}

.stat-label {
    font-size: var(--font-size-sm);
    color: var(--text-secondary);
    font-weight: 500;
}

.stat-value {
    font-size: var(--font-size-lg);
    font-weight: 700;
    color: var(--text-primary);
}

.header-controls {
    position: relative;
    display: flex;
    justify-content: center;
    gap: var(--spacing-sm);
    z-index: 1000;
}

.control-btn {
    background: var(--bg-secondary);
    border: 2px solid var(--tile-border);
    border-radius: var(--radius-md);
    width: 48px;
    height: 48px;
    font-size: var(--font-size-xl);
    cursor: pointer;
    transition: all var(--transition-fast);
    display: flex;
    align-items: center;
    justify-content: center;
    color: var(--text-primary);
}

.control-btn:hover:not(:disabled) {
    transform: translateY(-2px);
    box-shadow: var(--shadow-md);
    border-color: var(--color-primary);
}

.control-btn:active:not(:disabled) {
    transform: translateY(0);
}

.control-btn:disabled {
    opacity: 0.4;
    cursor: not-allowed;
}

.pause-btn.paused {
    border-color: var(--color-error);
    color: var(--color-error);
}

/* ========================================
   MODE TOGGLE
   ======================================== */
.mode-toggle {
    display: flex;
    gap: var(--spacing-sm);
    margin-bottom: var(--spacing-lg);
    background: var(--bg-secondary);
    padding: var(--spacing-xs);
    border-radius: var(--radius-md);
}

.mode-btn {
    flex: 1;
    padding: var(--spacing-sm) var(--spacing-md);
    border: none;
    background: transparent;
    color: var(--text-secondary);
    font-weight: 600;
    font-size: var(--font-size-base);
    border-radius: var(--radius-sm);
    cursor: pointer;
    transition: all var(--transition-fast);
}

.mode-btn:hover {
    background: var(--tile-hover);
}

.mode-btn.active {
    background: var(--color-primary);
    color: white;
    box-shadow: var(--shadow-sm);
}

/* ========================================
   DIFFICULTY SELECTOR
   ======================================== */
.difficulty-selector {
    text-align: center;
    padding: var(--spacing-xl) 0;
}

.difficulty-selector h2 {
    font-size: var(--font-size-xl);
    margin-bottom: var(--spacing-lg);
    color: var(--text-primary);
}

.difficulty-buttons {
    display: flex;
    flex-direction: column;
    gap: var(--spacing-md);
}

.difficulty-btn {
    padding: var(--spacing-md) var(--spacing-xl);
    font-size: var(--font-size-lg);
    font-weight: 600;
    border: 2px solid var(--tile-border);
    background: var(--bg-secondary);
    color: var(--text-primary);
    border-radius: var(--radius-md);
    cursor: pointer;
    transition: all var(--transition-normal);
}

.difficulty-btn:hover {
    transform: translateY(-2px);
    box-shadow: var(--shadow-md);
    border-color: var(--color-primary);
    background: var(--color-primary);
    color: white;
}

/* ========================================
   BOARD CONTAINER
   ======================================== */
.board-container {
    width: 100%;
    max-width: 450px;
    aspect-ratio: 1;
    margin: 0 auto var(--spacing-lg);
    display: grid;
    grid-template-columns: repeat(9, 1fr);
    grid-template-rows: repeat(9, 1fr);
    gap: 0;
    border: 3px solid var(--tile-border-thick);
    border-radius: var(--radius-md);
    overflow: hidden;
    background: var(--tile-border);
    box-shadow: var(--shadow-md);
    position: relative;
}

/* ========================================
   BOARD TILES
   ======================================== */
.board-tile {
    background: var(--tile-bg);
    border: 1px solid var(--tile-border);
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: clamp(14px, 3vw, 20px);
    font-weight: 700;
    cursor: pointer;
    transition: all var(--transition-fast);
    position: relative;
    color: var(--text-primary);
}

.board-tile:hover:not(.starter-tile) {
    /* background: var(--tile-hover); */
    transform: scale(1.05);
    z-index: 1;
}

.board-tile.selected {
    background: var(--tile-selected);
    box-shadow: inset 0 0 0 2px var(--color-primary);
    z-index: 2;
}

.board-tile.highlighted {
    background: var(--tile-highlight);
}

.board-tile.conflict {
    background: var(--tile-conflict);
    animation: shake 0.3s ease-in-out;
}

.starter-tile {
    background: var(--tile-starter);
    cursor: default;
    color: var(--text-primary);
}

.starter-tile:hover {
    transform: none;
}

.correct-digit {
    color: var(--color-primary);
    animation: popIn 0.3s ease-out;
}

.incorrect-digit {
    color: var(--color-error);
    animation: shake 0.3s ease-in-out;
}

/* Notes styling */
.board-tile .notes {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    grid-template-rows: repeat(3, 1fr);
    width: 100%;
    height: 100%;
    padding: 2px;
    gap: 1px;
}

.board-tile .note {
    font-size: clamp(8px, 1.2vw, 10px);
    font-weight: 500;
    color: var(--text-secondary);
    display: flex;
    align-items: center;
    justify-content: center;
}

/* Grid lines */
.horizontal-line {
    border-bottom: 3px solid var(--tile-border-thick);
}

.vertical-line {
    border-right: 3px solid var(--tile-border-thick);
}

/* ========================================
   PAUSE OVERLAY
   ======================================== */
.pause-overlay {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background: rgba(0, 0, 0, 0.9);
    backdrop-filter: blur(10px);
    display: none;
    align-items: center;
    justify-content: center;
    border-radius: var(--radius-md);
    z-index: 10;
}

.pause-overlay.active {
    display: flex;
    animation: fadeIn 0.3s ease-out;
}

.pause-content {
    text-align: center;
    color: white;
}

.pause-content h2 {
    font-size: var(--font-size-2xl);
    margin-bottom: var(--spacing-sm);
}

.pause-content p {
    font-size: var(--font-size-lg);
    opacity: 0.8;
}

/* ========================================
   CHOICE CONTAINER
   ======================================== */
.choice-container {
    display: grid;
    grid-template-columns: repeat(9, 1fr);
    gap: var(--spacing-sm);
    margin-bottom: var(--spacing-lg);
    max-width: 450px;
    margin-left: auto;
    margin-right: auto;
}

.choice-tile {
    aspect-ratio: 1;
    min-width: 0;
    background: var(--bg-secondary);
    border: 2px solid var(--tile-border);
    border-radius: var(--radius-sm);
    font-size: var(--font-size-xl);
    font-weight: 700;
    color: var(--color-primary);
    cursor: pointer;
    transition: all var(--transition-fast);
    display: flex;
    align-items: center;
    justify-content: center;
}

.choice-tile:hover {
    transform: translateY(-2px);
    box-shadow: var(--shadow-md);
    border-color: var(--color-primary);
}

.choice-tile.selected {
    background: var(--color-primary);
    color: white;
    border-color: var(--color-primary);
    box-shadow: var(--shadow-glow);
}

.choice-tile.disabled {
    opacity: 0.3;
    cursor: not-allowed;
}

.choice-tile.disabled:hover {
    transform: none;
    box-shadow: none;
}

/* ========================================
   ACTION BUTTONS
   ======================================== */
.action-buttons {
    display: flex;
    gap: var(--spacing-sm);
    margin-bottom: var(--spacing-lg);
}

.action-btn {
    flex: 1;
    padding: var(--spacing-md) var(--spacing-lg);
    font-size: var(--font-size-base);
    font-weight: 600;
    border: none;
    border-radius: var(--radius-md);
    cursor: pointer;
    transition: all var(--transition-fast);
    font-family: var(--font-family);
}

.action-btn.primary {
    background: var(--color-primary);
    color: white;
}

.action-btn.primary:hover {
    background: var(--color-primary-dark);
    transform: translateY(-2px);
    box-shadow: var(--shadow-md);
}

.action-btn.secondary {
    background: var(--bg-secondary);
    color: var(--text-primary);
    border: 2px solid var(--tile-border);
}

.action-btn.secondary:hover {
    border-color: var(--color-primary);
    transform: translateY(-2px);
    box-shadow: var(--shadow-md);
}

/* ========================================
   INSTRUCTIONS
   ======================================== */
.instructions {
    background: var(--bg-secondary);
    padding: var(--spacing-lg);
    border-radius: var(--radius-md);
    border: 2px solid var(--tile-border);
}

.instructions h3 {
    font-size: var(--font-size-lg);
    margin-bottom: var(--spacing-md);
    color: var(--text-primary);
}

.instructions ul {
    list-style: none;
    display: flex;
    flex-direction: column;
    gap: var(--spacing-sm);
}

.instructions li {
    padding-left: var(--spacing-md);
    position: relative;
    color: var(--text-secondary);
    font-size: var(--font-size-sm);
}

.instructions li::before {
    content: "•";
    position: absolute;
    left: 0;
    color: var(--color-primary);
    font-weight: bold;
}

.instructions strong {
    color: var(--text-primary);
}

/* ========================================
   WIN MODAL
   ======================================== */
.win-modal {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background: rgba(0, 0, 0, 0.8);
    backdrop-filter: blur(10px);
    display: none;
    align-items: center;
    justify-content: center;
    z-index: 1000;
}

.win-modal.active {
    display: flex;
    animation: fadeIn 0.3s ease-out;
}

.win-content {
    background: var(--bg-card);
    padding: var(--spacing-xl);
    border-radius: var(--radius-xl);
    text-align: center;
    box-shadow: var(--shadow-lg);
    max-width: 400px;
    animation: scaleIn 0.5s cubic-bezier(0.34, 1.56, 0.64, 1);
}

.win-content h2 {
    font-size: var(--font-size-2xl);
    margin-bottom: var(--spacing-md);
    color: var(--color-success);
}

.win-content p {
    font-size: var(--font-size-lg);
    color: var(--text-secondary);
    margin-bottom: var(--spacing-sm);
}

.final-time {
    font-size: 3rem !important;
    font-weight: 700 !important;
    color: var(--color-primary) !important;
    margin-bottom: var(--spacing-lg) !important;
}

/* ========================================
   CONFETTI
   ======================================== */
#confetti-container {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    pointer-events: none;
    z-index: 999;
}

.confetti {
    position: absolute;
    width: 10px;
    height: 10px;
    background: var(--color-primary);
    animation: confettiFall 3s linear forwards;
}

/* ========================================
   ANIMATIONS
   ======================================== */
@keyframes fadeIn {
    from {
        opacity: 0;
    }

    to {
        opacity: 1;
    }
}

@keyframes scaleIn {
    from {
        opacity: 0;
        transform: scale(0.8);
    }

    to {
        opacity: 1;
        transform: scale(1);
    }
}

@keyframes popIn {
    0% {
        transform: scale(0.8);
    }

    50% {
        transform: scale(1.2);
    }

    100% {
        transform: scale(1);
    }
}

@keyframes shake {

    0%,
    100% {
        transform: translateX(0);
    }

    25% {
        transform: translateX(-5px);
    }

    75% {
        transform: translateX(5px);
    }
}

@keyframes confettiFall {
    to {
        transform: translateY(100vh) rotate(360deg);
        opacity: 0;
    }
}

/* ========================================
   RESPONSIVE DESIGN
   ======================================== */
@media (max-width: 600px) {
    body {
        padding: var(--spacing-sm);
    }

    .game-container {
        padding: var(--spacing-lg);
    }

    .game-title {
        font-size: var(--font-size-xl);
    }

    .header-stats {
        gap: var(--spacing-xs);
    }

    .stat-label {
        font-size: 0.75rem;
    }

    .stat-value {
        font-size: var(--font-size-base);
    }

    .control-btn {
        width: 40px;
        height: 40px;
        font-size: var(--font-size-lg);
    }

    .instructions {
        padding: var(--spacing-md);
    }

    .instructions li {
        font-size: 0.75rem;
    }
}

@media (max-width: 400px) {
    .header-stats {
        flex-direction: column;
        gap: var(--spacing-sm);
    }

    .stat-item {
        flex-direction: row;
        justify-content: space-between;
        width: 100%;
    }
}