:root {
    --bg-color: #121212;
    --container-bg: #1e1e1e;
    --text-primary: #e0e0e0;
    --text-secondary: #9e9e9e;
    --border-light: #333333;
    --border-heavy: #757575;
    
    --cell-bg: #2d2d2d;
    --cell-fixed-text: #ffffff;
    --cell-input-text: #64b5f6;
    
    --cell-hover: #3d3d3d;
    --cell-selected: #42a5f5;
    --cell-selected-text: #ffffff;
    
    --cell-highlight: #1a3c5a; /* Same row/col/box */
    --cell-same-number: #28547c; /* Same number highlight */
    
    --error-bg: #4a1c1f;
    --error-text: #ff5252;
    --correct-bg: #1b3d22;
    --correct-text: #4caf50;
    
    --btn-primary: #42a5f5;
    --btn-primary-hover: #1e88e5;
    --btn-secondary: #424242;
    --btn-secondary-hover: #616161;
}

* {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
}

body {
    font-family: 'Inter', sans-serif;
    background-color: var(--bg-color);
    color: var(--text-primary);
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    user-select: none;
}

.game-container {
    background-color: var(--container-bg);
    padding: 2rem;
    border-radius: 12px;
    box-shadow: 0 8px 24px rgba(0, 0, 0, 0.5);
    display: flex;
    flex-direction: column;
    gap: 1.5rem;
    max-width: 500px;
    width: 100%;
}

header {
    display: flex;
    justify-content: space-between;
    align-items: center;
}

h1 {
    font-size: 2rem;
    font-weight: 700;
    letter-spacing: 1px;
}

.stats {
    display: flex;
    gap: 1.5rem;
}

.stat-box {
    display: flex;
    flex-direction: column;
    align-items: flex-end;
}

.label {
    font-size: 0.75rem;
    color: var(--text-secondary);
    text-transform: uppercase;
    letter-spacing: 0.5px;
}

#mistakes, #timer {
    font-size: 1.25rem;
    font-weight: 600;
    font-variant-numeric: tabular-nums;
}

.controls-top {
    display: flex;
    justify-content: space-between;
    align-items: center;
}

select {
    background-color: var(--btn-secondary);
    color: var(--text-primary);
    border: 1px solid var(--border-light);
    padding: 0.5rem 1rem;
    border-radius: 6px;
    font-family: inherit;
    font-size: 1rem;
    cursor: pointer;
    outline: none;
}

select:focus {
    border-color: var(--btn-primary);
}

.btn {
    border: none;
    padding: 0.75rem 1.25rem;
    border-radius: 6px;
    font-family: inherit;
    font-size: 1rem;
    font-weight: 500;
    cursor: pointer;
    transition: background-color 0.2s ease, transform 0.1s ease;
}

.btn:active {
    transform: scale(0.97);
}

.btn.primary {
    background-color: var(--btn-primary);
    color: white;
}

.btn.primary:hover {
    background-color: var(--btn-primary-hover);
}

.btn.secondary {
    background-color: var(--btn-secondary);
    color: var(--text-primary);
}

.btn.secondary:hover {
    background-color: var(--btn-secondary-hover);
}

/* Sudoku Board Layout using CSS Grid */
.sudoku-board {
    display: grid;
    grid-template-columns: repeat(9, 1fr);
    gap: 1px;
    background-color: var(--border-light);
    border: 3px solid var(--border-heavy);
    aspect-ratio: 1 / 1;
    width: 100%;
}

.cell {
    background-color: var(--cell-bg);
    display: flex;
    justify-content: center;
    align-items: center;
    font-size: 1.75rem;
    font-weight: 500;
    cursor: pointer;
    color: var(--cell-input-text);
    transition: background-color 0.15s ease, color 0.15s ease;
}

/* 3x3 Grid Thicker Borders */
/* We use border-right and border-bottom on specific cells to create the 3x3 separation */
.cell:nth-child(3n) {
    border-right: 2px solid var(--border-heavy);
}
.cell:nth-child(9n) {
    border-right: none;
}
.cell:nth-child(n+19):nth-child(-n+27),
.cell:nth-child(n+46):nth-child(-n+54) {
    border-bottom: 2px solid var(--border-heavy);
}

/* Cell States */
.cell.fixed {
    color: var(--cell-fixed-text);
    font-weight: 600;
}

.cell:hover:not(.selected) {
    background-color: var(--cell-hover);
}

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

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

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

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

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

.numpad {
    display: grid;
    grid-template-columns: repeat(5, 1fr);
    gap: 0.5rem;
}

.num-btn {
    background-color: var(--btn-secondary);
    color: var(--text-primary);
    border: none;
    border-radius: 8px;
    padding: 1rem 0;
    font-size: 1.5rem;
    font-weight: 600;
    cursor: pointer;
    transition: background-color 0.1s ease, transform 0.1s ease;
}

.num-btn:hover {
    background-color: var(--btn-secondary-hover);
}

.num-btn:active {
    transform: scale(0.95);
}

.action-icon {
    font-size: 1.25rem;
    color: var(--error-text);
}

.controls-bottom {
    display: grid;
    grid-template-columns: 1fr 1fr 1fr;
    gap: 0.75rem;
}

#new-game-btn {
    grid-column: span 3;
}

/* Modals */
.modal-overlay {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background-color: rgba(0, 0, 0, 0.7);
    display: flex;
    justify-content: center;
    align-items: center;
    z-index: 100;
    opacity: 1;
    transition: opacity 0.3s ease;
}

.modal-overlay.hidden {
    opacity: 0;
    pointer-events: none;
}

.modal {
    background-color: var(--container-bg);
    padding: 2.5rem;
    border-radius: 12px;
    text-align: center;
    box-shadow: 0 10px 30px rgba(0, 0, 0, 0.5);
    max-width: 90%;
    width: 400px;
}

.modal h2 {
    font-size: 2rem;
    margin-bottom: 1rem;
}

.modal.success h2 {
    color: var(--correct-text);
}

.modal.fail h2 {
    color: var(--error-text);
}

.modal p {
    color: var(--text-secondary);
    margin-bottom: 2rem;
    font-size: 1.1rem;
}

@media (max-width: 480px) {
    .game-container {
        padding: 1rem;
        gap: 1rem;
        border-radius: 0;
        min-height: 100vh;
        justify-content: space-evenly;
    }
    
    .cell {
        font-size: 1.25rem;
    }
    
    .numpad {
        grid-template-columns: repeat(5, 1fr);
    }
    
    .num-btn {
        padding: 0.75rem 0;
        font-size: 1.25rem;
    }
}