:root {
    --bg-color: #f5f5f5;
    --text-color: #333;
    --cell-color: #fff;
    --border-color: #ddd;
    --highlight-color: #e6f7ff;
    --selected-color: #b3e0ff;
    --error-color: #ffcccc;
    --number-highlight: #d4e6f1;
    --thick-border-color: #999;
    --button-bg: #4a6fa5;
    --button-text: #fff;
    --disabled-button: #cccccc;
    --modal-bg: rgba(0, 0, 0, 0.7);
    --given-cell-color: #e0e0e0;
    --correct-highlight: #d4e6f1;
}

.dark-mode {
    --bg-color: #2d2d2d;
    --text-color: #f0f0f0;
    --cell-color: #3d3d3d;
    --border-color: #555;
    --highlight-color: #334d4d;
    --selected-color: #2a4d4d;
    --error-color: #660000;
    --number-highlight: #334d5c;
    --thick-border-color: #777;
    --button-bg: #5a8faf;
    --button-text: #f0f0f0;
    --disabled-button: #555555;
    --modal-bg: rgba(0, 0, 0, 0.9);
    --given-cell-color: #555555;
    --correct-highlight: #334d5c;
}

* {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
}

body {
    background-color: var(--bg-color);
    color: var(--text-color);
    transition: background-color 0.3s, color 0.3s;
    padding: 20px;
    display: flex;
    flex-direction: column;
    align-items: center;
    min-height: 100vh;
}

header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    width: 100%;
    max-width: 600px;
    margin-bottom: 20px;
}

h1 {
    font-size: 1.8rem;
    color: var(--text-color);
}

.controls {
    display: flex;
    gap: 10px;
    margin-bottom: 20px;
    flex-wrap: wrap;
    justify-content: center;
    width: 100%;
    max-width: 600px;
}

button {
    background-color: var(--button-bg);
    color: var(--button-text);
    border: none;
    padding: 8px 15px;
    border-radius: 5px;
    cursor: pointer;
    font-size: 0.9rem;
    transition: background-color 0.2s;
}

button:hover {
    opacity: 0.9;
}

button:disabled {
    background-color: var(--disabled-button);
    cursor: not-allowed;
}

.dropdown {
    position: relative;
    display: inline-block;
}

.dropdown-content {
    display: none;
    position: absolute;
    background-color: var(--cell-color);
    min-width: 120px;
    box-shadow: 0px 8px 16px 0px rgba(0,0,0,0.2);
    z-index: 1;
    border-radius: 5px;
    overflow: hidden;
}

.dropdown-content button {
    width: 100%;
    text-align: left;
    background-color: var(--cell-color);
    color: var(--text-color);
    border-radius: 0;
    border-bottom: 1px solid var(--border-color);
}

.dropdown-content button:hover {
    background-color: var(--highlight-color);
}

.dropdown:hover .dropdown-content {
    display: block;
}

.stats {
    display: flex;
    gap: 20px;
    margin-bottom: 20px;
    font-size: 1rem;
}

.board-container {
    width: 100%;
    max-width: 600px;
    aspect-ratio: 1/1;
    margin-bottom: 20px;
}

#sudoku-board {
    width: 100%;
    height: 100%;
    border-collapse: collapse;
    background-color: var(--cell-color);
}

#sudoku-board td {
    border: 1px solid var(--border-color);
    position: relative;
    width: 11.11%;
    height: 11.11%;
    text-align: center;
    font-size: 1.5rem;
    font-weight: bold;
    cursor: pointer;
    transition: background-color 0.2s;
}

#sudoku-board td:hover {
    background-color: var(--highlight-color);
}

#sudoku-board td.selected {
    background-color: var(--selected-color);
    border: 2px solid var(--button-bg);
}

#sudoku-board td.error {
    color: #ff0000;
    background-color: var(--error-color);
}

#sudoku-board td.given {
    background-color: var(--given-cell-color);
    color: var(--text-color);
    font-weight: bold;
}

#sudoku-board td[data-row]:nth-child(3n) {
    border-right: 2px solid var(--thick-border-color);
}

#sudoku-board tr[data-row]:nth-child(3n) td {
    border-bottom: 2px solid var(--thick-border-color);
}

.notes {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    grid-template-rows: repeat(3, 1fr);
    font-size: 0.6rem;
    color: var(--text-color);
    opacity: 0.7;
    pointer-events: none;
}

.note-number {
    display: flex;
    align-items: center;
    justify-content: center;
}

.number-buttons {
    display: flex;
    justify-content: center;
    gap: 5px;
    width: 100%;
    max-width: 600px;
    margin-bottom: 20px;
}

.number-button {
    width: 40px;
    height: 50px;
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    background-color: var(--cell-color);
    border: 1px solid var(--border-color);
    border-radius: 5px;
    cursor: pointer;
    transition: background-color 0.2s;
}

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

.number-button .main-number {
    font-size: 1.2rem;
    font-weight: bold;
}

.number-button .remaining-count {
    font-size: 0.6rem;
    opacity: 0.7;
}

.modal {
    display: none;
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background-color: var(--modal-bg);
    z-index: 100;
    justify-content: center;
    align-items: center;
}

.modal-content {
    background-color: var(--cell-color);
    padding: 30px;
    border-radius: 10px;
    text-align: center;
    max-width: 80%;
    box-shadow: 0 5px 15px rgba(0,0,0,0.3);
}

.modal h2 {
    margin-bottom: 20px;
    color: var(--text-color);
}

.modal p {
    margin-bottom: 20px;
    font-size: 1.1rem;
}

.highlight-number {
    background-color: var(--correct-highlight);
}

.highlight-row-col {
    background-color: var(--highlight-color);
}

@media (max-width: 600px) {
    h1 {
        font-size: 1.5rem;
    }
    
    .controls {
        flex-direction: column;
        align-items: center;
    }
    
    button {
        padding: 6px 12px;
        font-size: 0.8rem;
    }
    
    #sudoku-board td {
        font-size: 1.2rem;
    }
    
    .number-button {
        width: 30px;
        height: 40px;
    }
    
    .number-button .main-number {
        font-size: 1rem;
    }
}