:root {
    --board-size: 540px;
    --background-color: #121212;
    --board-bg: #282828;
    --cell-bg: #3a3a3a;
    --text-color: #ffffff;
    --border-color: #444;
    --border-color-bold: #666;
    --accent-color: #4a90e2;
    --error-color: #d0021b;
    --selected-bg: #4a90e2;
    --highlight-bg: #3f4a55;
    --given-text-color: #e0e0e0;
    --user-text-color: var(--accent-color);
}

body {
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    background-color: var(--background-color);
    color: var(--text-color);
    margin: 0;
    padding: 20px;
}

.content-wrapper {
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
}

#game-container {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 25px;
}

header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    width: var(--board-size);
}

h1 {
    font-size: 3rem;
    font-weight: bold;
}

.game-controls {
    display: flex;
    align-items: center;
    gap: 20px;
}

.difficulty-selector select, #new-game-btn {
    background-color: var(--cell-bg);
    color: var(--text-color);
    border: 1px solid var(--border-color);
    border-radius: 5px;
    padding: 8px 12px;
    font-size: 1rem;
}

#new-game-btn {
    cursor: pointer;
    background-color: var(--accent-color);
    border-color: var(--accent-color);
    font-weight: bold;
}

#new-game-btn:hover {
    opacity: 0.9;
}

main {
    width: var(--board-size);
    display: flex;
    flex-direction: column;
    gap: 20px;
}

#stats-container {
    display: flex;
    justify-content: space-around;
    width: 100%;
}

.stat-box {
    background-color: var(--board-bg);
    padding: 10px 20px;
    border-radius: 8px;
    text-align: center;
}

.stat-box h3 {
    margin: 0 0 5px;
    font-size: 1rem;
    color: #ccc;
}

.stat-box p {
    margin: 0;
    font-size: 1.5rem;
    font-weight: bold;
}

#sudoku-board {
    display: grid;
    grid-template-columns: repeat(9, 1fr);
    grid-template-rows: repeat(9, 1fr);
    width: var(--board-size);
    height: var(--board-size);
    background-color: var(--board-bg);
    border: 3px solid var(--border-color-bold);
    border-radius: 8px;
    gap: 1px;
    padding: 2px;
}

.sudoku-cell {
    background-color: var(--cell-bg);
    display: flex;
    justify-content: center;
    align-items: center;
    font-size: 2rem;
    font-weight: bold;
    cursor: pointer;
    border: 1px solid var(--border-color);
}

.sudoku-cell:nth-child(3n) { border-right-color: var(--border-color-bold); }
.sudoku-cell:nth-child(9n) { border-right: none; }
.sudoku-cell:nth-child(n+19):nth-child(-n+27),
.sudoku-cell:nth-child(n+46):nth-child(-n+54) {
    border-bottom-color: var(--border-color-bold);
}

.sudoku-cell.given {
    color: var(--given-text-color);
    cursor: default;
}

.sudoku-cell.user-filled {
    color: var(--user-text-color);
}

.sudoku-cell.highlighted {
    background-color: var(--highlight-bg);
}

.sudoku-cell.error {
    color: var(--error-color) !important;
    animation: shake 0.3s;
}

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

#number-palette {
    display: grid;
    grid-template-columns: repeat(9, 1fr);
    gap: 10px;
}

.number-option {
    background-color: var(--cell-bg);
    border: 1px solid var(--border-color);
    border-radius: 8px;
    height: 50px;
    display: flex;
    justify-content: center;
    align-items: center;
    font-size: 1.8rem;
    font-weight: bold;
    cursor: pointer;
}

.number-option:hover {
    background-color: var(--highlight-bg);
}

.hidden {
    display: none !important;
}

#game-over-message {
    position: fixed;
    top: 0; left: 0; right: 0; bottom: 0;
    background-color: rgba(18, 18, 18, 0.85);
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    text-align: center;
    z-index: 100;
}

#game-over-message h2 {
    font-size: 3rem;
    margin: 0;
}

#game-over-message p {
    font-size: 1.2rem;
    margin-top: 10px;
}

#restart-button {
    margin-top: 20px;
    padding: 12px 25px;
    font-size: 1.2rem;
    border: none;
    border-radius: 5px;
    background-color: var(--accent-color);
    color: white;
    cursor: pointer;
    font-weight: bold;
}

#instructions {
    margin-top: 30px;
    max-width: var(--board-size);
    background-color: var(--board-bg);
    padding: 20px;
    border-radius: 8px;
}

#instructions h2 {
    margin-top: 0;
    color: var(--text-color);
}

#instructions ul {
    padding-left: 20px;
    margin: 10px 0 0;
    color: #ccc;
}

#instructions li {
    margin-bottom: 10px;
    line-height: 1.4;
}