:root {
    --bg-color: #f0f2f5;
    --container-bg: #fff;
    --primary-color: #4CAF50;
    --secondary-color: #2196F3;
    --text-color: #333;
    --border-color: #ccc;
    --grid-border-color: #555;
    --fixed-cell-bg: #e8e8e8;
    --hover-cell-bg: #f5f5f5;
    --selected-cell-bg: #bbdefb;
    --highlight-same-bg: #e3f2fd;
    --drag-over-cell-bg: #a5d6a7;
    --palette-bg: #f9f9f9;
    --palette-number-bg: #e0e0e0;
    --palette-number-hover-bg: #ccc;
    --error-color: #f44336;
    --success-color: #4CAF50;
    --notes-color: #888;
    --shadow: 0 4px 16px rgba(0,0,0,0.10);
}

body.dark {
    --bg-color: #1a1a2e;
    --container-bg: #16213e;
    --text-color: #e0e0e0;
    --border-color: #444;
    --grid-border-color: #888;
    --fixed-cell-bg: #0f3460;
    --hover-cell-bg: #1e2a45;
    --selected-cell-bg: #1565c0;
    --highlight-same-bg: #1a3a5c;
    --drag-over-cell-bg: #2e7d32;
    --palette-bg: #0f3460;
    --palette-number-bg: #1e2a45;
    --palette-number-hover-bg: #2a3a5a;
    --shadow: 0 4px 16px rgba(0,0,0,0.4);
}

* { box-sizing: border-box; }

body {
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    display: flex;
    justify-content: center;
    align-items: flex-start;
    min-height: 100vh;
    background-color: var(--bg-color);
    margin: 0;
    color: var(--text-color);
    padding: 20px;
    transition: background-color 0.3s, color 0.3s;
}

.game-container {
    background-color: var(--container-bg);
    border-radius: 14px;
    box-shadow: var(--shadow);
    padding: 28px 24px;
    text-align: center;
    max-width: 560px;
    width: 100%;
    transition: background-color 0.3s;
    position: relative;
}

/* ── Top Bar ── */
.top-bar {
    display: flex;
    justify-content: center;
    align-items: center;
    position: relative;
    margin-bottom: 18px;
}

h1 {
    color: var(--primary-color);
    font-size: 2.4em;
    letter-spacing: 2px;
    margin: 0;
}

.icon-btn {
    position: absolute;
    right: 0;
    background: none;
    border: none;
    font-size: 1.5em;
    cursor: pointer;
    padding: 4px 8px;
    border-radius: 8px;
    transition: background 0.2s;
}
.icon-btn:hover { background: var(--hover-cell-bg); }

/* ── Controls ── */
.controls {
    display: flex;
    flex-wrap: wrap;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 14px;
    gap: 10px;
}

.difficulty-selection, .game-info {
    display: flex;
    gap: 8px;
    align-items: center;
}

.difficulty-btn, .control-btn {
    padding: 9px 16px;
    border: none;
    border-radius: 8px;
    cursor: pointer;
    font-size: 0.95em;
    font-weight: 600;
    transition: background-color 0.2s, transform 0.1s;
    box-shadow: 0 2px 4px rgba(0,0,0,0.1);
}

.difficulty-btn {
    background-color: var(--palette-number-bg);
    color: var(--text-color);
}
.difficulty-btn:hover { background-color: var(--palette-number-hover-bg); }
.difficulty-btn.active {
    background-color: var(--primary-color);
    color: #fff;
    box-shadow: 0 3px 8px rgba(0,0,0,0.2);
}

.control-btn {
    background-color: var(--secondary-color);
    color: #fff;
}
.control-btn:hover {
    background-color: #1565c0;
    transform: translateY(-1px);
}
.control-btn.active {
    background-color: #ff9800;
}

/* ── Action Bar ── */
.action-bar {
    display: flex;
    flex-wrap: wrap;
    justify-content: center;
    gap: 8px;
    margin-bottom: 10px;
}

.action-bar .control-btn {
    font-size: 0.88em;
    padding: 7px 12px;
}

/* ── Timer & Mistakes ── */
.timer {
    font-size: 1.4em;
    font-weight: bold;
    color: var(--primary-color);
    background-color: var(--palette-number-bg);
    padding: 6px 14px;
    border-radius: 8px;
    min-width: 80px;
    text-align: center;
}

.mistake-counter {
    font-size: 1em;
    font-weight: bold;
    background-color: var(--palette-number-bg);
    padding: 6px 12px;
    border-radius: 8px;
    color: var(--text-color);
    transition: background 0.3s, color 0.3s;
}
.mistake-counter.danger {
    background-color: #ffebee;
    color: var(--error-color);
}
body.dark .mistake-counter.danger {
    background-color: #4a1010;
}

/* ── Grid ── */
.sudoku-grid {
    display: grid;
    grid-template-columns: repeat(9, 1fr);
    grid-template-rows: repeat(9, 1fr);
    width: 100%;
    max-width: 450px;
    aspect-ratio: 1 / 1;
    margin: 16px auto;
    border: 3px solid var(--grid-border-color);
    box-shadow: var(--shadow);
    border-radius: 6px;
    overflow: hidden;
    position: relative;
}

.sudoku-cell {
    width: 100%;
    height: 100%;
    display: flex;
    justify-content: center;
    align-items: center;
    font-size: 1.5em;
    font-weight: bold;
    border: 1px solid var(--border-color);
    cursor: pointer;
    transition: background-color 0.1s;
    user-select: none;
    background-color: var(--container-bg);
    color: var(--text-color);
    position: relative;
}

/* 3x3 block borders */
.sudoku-cell:nth-child(3n):not(:nth-child(9n)) { border-right: 2px solid var(--grid-border-color); }
.sudoku-cell:nth-child(n+19):nth-child(-n+27),
.sudoku-cell:nth-child(n+46):nth-child(-n+54) { border-bottom: 2px solid var(--grid-border-color); }

.sudoku-cell.fixed {
    background-color: var(--fixed-cell-bg);
    cursor: default;
}
.sudoku-cell.user-filled { color: var(--secondary-color); }
.sudoku-cell.error {
    background-color: #ffebee;
    color: var(--error-color);
}
body.dark .sudoku-cell.error { background-color: #4a1010; }

.sudoku-cell.selected { background-color: var(--selected-cell-bg) !important; }
.sudoku-cell.highlight-same { background-color: var(--highlight-same-bg); }
.sudoku-cell.drag-over {
    background-color: var(--drag-over-cell-bg);
    border: 2px dashed var(--primary-color);
}
.sudoku-cell:not(.fixed):hover { background-color: var(--hover-cell-bg); }

/* ── Notes Grid ── */
.sudoku-cell.notes-cell { font-size: 0.5em; }
.note-grid {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    grid-template-rows: repeat(3, 1fr);
    width: 100%;
    height: 100%;
    padding: 1px;
}
.note-grid span {
    display: flex;
    justify-content: center;
    align-items: center;
    font-size: 0.75em;
    color: var(--notes-color);
    font-weight: normal;
    line-height: 1;
}

/* ── Pause Overlay ── */
.pause-overlay {
    position: absolute;
    top: 0; left: 0; right: 0; bottom: 0;
    background: rgba(0,0,0,0.65);
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    gap: 16px;
    border-radius: 6px;
    z-index: 10;
}
.pause-message {
    color: #fff;
    font-size: 2em;
    font-weight: bold;
}

/* ── Number Palette ── */
.number-palette {
    display: flex;
    justify-content: center;
    gap: 8px;
    margin-top: 16px;
    background-color: var(--palette-bg);
    padding: 12px;
    border-radius: 10px;
    box-shadow: 0 2px 8px rgba(0,0,0,0.06);
    flex-wrap: wrap;
}

.palette-number {
    width: 44px;
    height: 44px;
    display: flex;
    justify-content: center;
    align-items: center;
    background-color: var(--palette-number-bg);
    border-radius: 8px;
    font-size: 1.3em;
    font-weight: bold;
    cursor: grab;
    transition: background-color 0.2s, transform 0.1s;
    box-shadow: 0 2px 4px rgba(0,0,0,0.1);
    color: var(--text-color);
}
.palette-number:hover {
    background-color: var(--palette-number-hover-bg);
    transform: translateY(-2px);
}
.palette-number:active { cursor: grabbing; transform: translateY(0); }
.palette-number.clear-number { background-color: var(--error-color); color: #fff; }
.palette-number.clear-number:hover { background-color: #c62828; }
.palette-number.dragging { opacity: 0.5; }

/* ── Game Message ── */
.game-message {
    margin-top: 14px;
    font-size: 1.1em;
    font-weight: bold;
    min-height: 28px;
}
.game-message.success { color: var(--success-color); }
.game-message.error { color: var(--error-color); }

/* ── Win Overlay ── */
.win-overlay {
    position: fixed;
    inset: 0;
    background: rgba(0,0,0,0.6);
    display: flex;
    justify-content: center;
    align-items: center;
    z-index: 100;
}
.win-box {
    background: var(--container-bg);
    color: var(--text-color);
    border-radius: 16px;
    padding: 40px 50px;
    text-align: center;
    box-shadow: 0 8px 32px rgba(0,0,0,0.3);
    z-index: 101;
    position: relative;
}
.win-emoji { font-size: 3.5em; margin-bottom: 10px; }
.win-box h2 { font-size: 2em; color: var(--primary-color); margin: 0 0 16px; }
.win-box p { margin: 6px 0; font-size: 1.1em; }
.win-box .control-btn { margin-top: 20px; font-size: 1em; padding: 12px 28px; }

#confetti-canvas {
    position: fixed;
    inset: 0;
    pointer-events: none;
    z-index: 100;
}

/* ── Stats Modal ── */
.modal {
    position: fixed;
    inset: 0;
    background: rgba(0,0,0,0.55);
    display: flex;
    justify-content: center;
    align-items: center;
    z-index: 200;
}
.modal-box {
    background: var(--container-bg);
    color: var(--text-color);
    border-radius: 14px;
    padding: 32px 40px;
    min-width: 300px;
    text-align: center;
    box-shadow: 0 8px 32px rgba(0,0,0,0.25);
}
.modal-box h2 { margin: 0 0 20px; color: var(--primary-color); }
.stat-block {
    background: var(--palette-bg);
    border-radius: 10px;
    padding: 14px 18px;
    margin-bottom: 12px;
    text-align: left;
}
.stat-block h3 {
    margin: 0 0 8px;
    text-transform: capitalize;
    color: var(--secondary-color);
}
.stat-block p { margin: 4px 0; font-size: 0.95em; }
.modal-box .control-btn { margin-top: 16px; }

/* ── Hidden ── */
.hidden { display: none !important; }

/* ── How to Play ── */
.how-to-play {
    margin-top: 20px;
    border-top: 1px solid var(--border-color);
    padding-top: 14px;
    text-align: left;
}

.how-to-play-toggle {
    width: 100%;
    background: var(--palette-bg);
    border: 1px solid var(--border-color);
    border-radius: 10px;
    padding: 12px 16px;
    font-size: 1em;
    font-weight: 600;
    color: var(--text-color);
    cursor: pointer;
    display: flex;
    justify-content: space-between;
    align-items: center;
    transition: background 0.2s;
}
.how-to-play-toggle:hover { background: var(--hover-cell-bg); }
.how-to-play-toggle .arrow { font-size: 0.8em; }

.how-to-play-content {
    margin-top: 10px;
    display: flex;
    flex-direction: column;
    gap: 12px;
}

.htp-section {
    background: var(--palette-bg);
    border-radius: 10px;
    padding: 14px 16px;
    border-left: 4px solid var(--primary-color);
}
.htp-section h3 {
    margin: 0 0 8px;
    font-size: 1em;
    color: var(--secondary-color);
}
.htp-section p, .htp-section ul {
    margin: 0;
    font-size: 0.92em;
    line-height: 1.6;
    color: var(--text-color);
}
.htp-section ul {
    padding-left: 18px;
}
.htp-section li { margin-bottom: 4px; }

/* ── Responsive ── */
@media (max-width: 600px) {
    .game-container { padding: 16px 12px; }
    h1 { font-size: 1.9em; }
    .controls { flex-direction: column; align-items: stretch; }
    .difficulty-selection, .game-info { justify-content: center; }
    .sudoku-grid { max-width: 320px; }
    .sudoku-cell { font-size: 1.1em; }
    .action-bar .control-btn { font-size: 0.82em; padding: 6px 10px; }
    .palette-number { width: 36px; height: 36px; font-size: 1.1em; }
    .win-box { padding: 28px 24px; }
    .modal-box { padding: 24px 18px; min-width: unset; width: 90%; }
}

@media (max-width: 380px) {
    .sudoku-grid { max-width: 270px; }
    .sudoku-cell { font-size: 0.95em; }
    .palette-number { width: 30px; height: 30px; font-size: 1em; }
}