:root {
    --bg-color: #f0f2f5;
    --grid-bg: #ffffff;
    --border-color: #1a1a1a;
    --primary-color: #4a90e2;
    --success-color: #52c41a;
    --error-color: #f5222d;
    --selected-color: #e6f7ff;
}

* { box-sizing: border-box; }

body {
    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
    background-color: var(--bg-color);
    display: flex;
    justify-content: center;
    align-items: flex-start;
    min-height: 100vh;
    margin: 0;
    padding: 10px;
}

.container {
    width: 100%;
    max-width: 450px;
    text-align: center;
    background: white;
    padding: 15px;
    border-radius: 16px;
    box-shadow: 0 4px 12px rgba(0,0,0,0.08);
}

h1 {
    font-size: 1.5rem;
    margin-bottom: 0.5rem;
    color: #333;
}

.stats {
    display: flex;
    justify-content: center;
    margin-bottom: 1rem;
    font-weight: bold;
    font-family: monospace;
    font-size: 1.2rem;
}

.grid {
    display: grid;
    grid-template-columns: repeat(9, 1fr);
    aspect-ratio: 1 / 1;
    border: 2px solid var(--border-color);
    margin: 0 auto;
    background-color: var(--border-color);
    user-select: none;
}

.cell {
    background: var(--grid-bg);
    border: 0.5px solid #dee2e6;
    display: flex;
    justify-content: center;
    align-items: center;
    cursor: pointer;
    font-size: 1.5rem;
    position: relative;
    transition: background 0.1s;
}

.cell .note-grid {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    grid-template-rows: repeat(3, 1fr);
    width: 100%;
    height: 100%;
    padding: 2px;
    pointer-events: none;
}

.cell .note-num {
    font-size: 0.6rem;
    color: #999;
    display: flex;
    justify-content: center;
    align-items: center;
}

.cell.selected {
    background-color: var(--selected-color);
}

.cell.related {
    background-color: #f0f7ff;
}

.cell.initial {
    background-color: #f5f5f5;
    font-weight: bold;
    color: #000;
}

.cell.wrong {
    color: var(--error-color);
}

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

/* 數字輸入盤 */
.number-pad {
    display: grid;
    grid-template-columns: repeat(5, 1fr);
    gap: 8px;
    margin: 20px 0;
}

.numpad-btn {
    padding: 12px 0;
    font-size: 1.2rem;
    background: #fff;
    border: 1px solid #ddd;
    border-radius: 8px;
    cursor: pointer;
    transition: all 0.2s;
}

.numpad-btn:active {
    background: var(--selected-color);
    transform: scale(0.95);
}

.numpad-btn.erase {
    grid-column: span 1;
    background: #fdf2f2;
    color: var(--error-color);
    font-size: 0.9rem;
}

.controls {
    display: flex;
    justify-content: space-between;
    gap: 10px;
}

button.main-btn {
    flex: 1;
    padding: 12px;
    font-size: 1rem;
    cursor: pointer;
    border: none;
    border-radius: 8px;
    color: white;
}

#new-game-btn { background-color: var(--primary-color); }
#check-btn { background-color: var(--success-color); }
.hint-btn { background-color: #faad14; }

#message {
    margin-top: 15px;
    font-size: 0.9rem;
    font-weight: bold;
    height: 1.2rem;
}

/* 手機版適配 */
@media (max-width: 400px) {
    .container { padding: 10px; }
    .numpad-btn { padding: 15px 0; }
}