:root {
    --bg-color: #f4f4f9;
    --container-bg: #ffffff;
    --text-color: #333333;
    --border-color: #333333;
    --cell-border-color: #cccccc;
    --initial-cell-bg: #e9ecef;
    --initial-cell-text: #343a40;
    --input-focus-bg: #d4e8ff;
    --user-input-color: #007bff;
    --incorrect-bg: #f8d7da;
    --incorrect-text: #721c24;
    --solution-color: #28a745;
    --shadow-color: rgba(0, 0, 0, 0.1);
}

body.dark-mode {
    --bg-color: #121212;
    --container-bg: #1e1e1e;
    --text-color: #e0e0e0;
    --border-color: #e0e0e0;
    --cell-border-color: #444444;
    --initial-cell-bg: #424242;
    --initial-cell-text: #e0e0e0;
    --input-focus-bg: #3a3d40;
    --user-input-color: #8ab4f8;
    --incorrect-bg: #5c2c33;
    --incorrect-text: #f5c6cb;
    --solution-color: #52d675;
    --shadow-color: rgba(255, 255, 255, 0.1);
}

body {
    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    margin: 0;
    background-color: var(--bg-color);
    color: var(--text-color);
    transition: background-color 0.3s, color 0.3s;
}

.container {
    width: 95%;
    max-width: 500px;
    text-align: center;
    background: var(--container-bg);
    padding: 20px;
    border-radius: 10px;
    box-shadow: 0 4px 15px var(--shadow-color);
    transition: background-color 0.3s;
}

.header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 20px;
}

h1 {
    font-size: 1.5em;
    margin: 0;
}

#sudoku-grid {
    border: 3px solid var(--border-color);
    border-collapse: collapse;
    margin: 0 auto;
    width: 100%;
    height: auto;
    aspect-ratio: 1 / 1;
}

.cell {
    border: 1px solid var(--cell-border-color);
    text-align: center;
    vertical-align: middle;
    font-size: clamp(16px, 4vw, 24px);
}

.initial-cell {
    background-color: var(--initial-cell-bg);
    font-weight: bold;
    color: var(--initial-cell-text);
}

.cell input {
    width: 100%;
    height: 100%;
    padding: 0;
    margin: 0;
    border: none;
    text-align: center;
    font-size: inherit;
    background-color: transparent;
    color: var(--user-input-color);
    font-weight: bold;
}

.cell input:focus {
    outline: none;
    background-color: var(--input-focus-bg);
}

.cell input.incorrect {
    background-color: var(--incorrect-bg);
    color: var(--incorrect-text);
}

.cell input.solution-cell {
    color: var(--solution-color);
    font-style: italic;
}

.cell:nth-of-type(3n) { border-right: 3px solid var(--border-color); }
#sudoku-grid tr:nth-of-type(3n) { border-bottom: 3px solid var(--border-color); }

.controls {
    margin-top: 25px;
    display: flex;
    flex-wrap: wrap;
    justify-content: space-around;
    align-items: center;
    gap: 15px;
}

select, button {
    padding: 10px 15px;
    font-size: 1em;
    border-radius: 5px;
    border: 1px solid var(--cell-border-color);
    cursor: pointer;
    background-color: var(--container-bg);
    color: var(--text-color);
}

button {
    background-color: var(--user-input-color);
    color: var(--container-bg);
    border-color: var(--user-input-color);
}

#solve-btn {
    background-color: var(--solution-color);
    border-color: var(--solution-color);
}

.theme-switch-wrapper {
    display: flex;
    align-items: center;
}
.theme-switch {
    position: relative;
    display: inline-block;
    width: 50px;
    height: 24px;
}
.theme-switch input {
    opacity: 0;
    width: 0;
    height: 0;
}
.slider {
    position: absolute;
    cursor: pointer;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background-color: #ccc;
    transition: .4s;
}
.slider:before {
    position: absolute;
    content: "";
    height: 18px;
    width: 18px;
    left: 3px;
    bottom: 3px;
    background-color: white;
    transition: .4s;
}
input:checked + .slider {
    background-color: var(--user-input-color);
}
input:checked + .slider:before {
    transform: translateX(26px);
}
.slider.round {
    border-radius: 34px;
}
.slider.round:before {
    border-radius: 50%;
}

@media (max-width: 480px) {
    .container {
        width: 100%;
        min-height: 100vh;
        border-radius: 0;
        padding: 10px;
        box-sizing: border-box;
    }

    h1 {
        font-size: 1.2em;
    }
    
    .controls {
        flex-direction: column;
    }
}