:root {
    /* Color Palette - Premium Dark/Blue Theme */
    --bg-color: #0f172a;
    --text-color: #f8fafc;
    --primary-color: #3b82f6;
    --secondary-color: #1e293b;
    --accent-color: #60a5fa;
    --highlight-color: #1e40af;
    --error-color: #ef4444;
    --success-color: #22c55e;
    --grid-border: #334155;
    --cell-bg: #1e293b;
    --cell-selected: #3b82f6;
    /* When user selects a cell */
    --cell-related: #1e3a8a;
    /* Row/Col/Box highlighting */
    --cell-same-num: #1d4ed8;
    /* Matching numbers */

    --font-family: 'Outfit', sans-serif;

    --border-radius: 12px;
    --shadow: 0 4px 6px -1px rgba(0, 0, 0, 0.1), 0 2px 4px -1px rgba(0, 0, 0, 0.06);
}

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    -webkit-tap-highlight-color: transparent;
}

body {
    font-family: var(--font-family);
    background-color: var(--bg-color);
    color: var(--text-color);
    height: 100vh;
    display: flex;
    justify-content: center;
    overflow: hidden;
}

.app-container {
    width: 100%;
    max-width: 500px;
    height: 100%;
    position: relative;
    background-color: #0f172a;
    display: flex;
    flex-direction: column;
}

.screen {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    display: flex;
    flex-direction: column;
    padding: 20px;
    transition: opacity 0.3s ease, transform 0.3s ease;
    background-color: var(--bg-color);
    z-index: 10;
}

.screen.hidden {
    opacity: 0;
    pointer-events: none;
    transform: scale(0.95);
    z-index: 0;
}

.screen.active {
    opacity: 1;
    pointer-events: all;
    transform: scale(1);
    z-index: 10;
}

/* --- Level Selector --- */
.main-header {
    text-align: center;
    margin-bottom: 20px;
}

.main-header h1 {
    font-size: 2.5rem;
    font-weight: 700;
    color: var(--primary-color);
}

.subtitle {
    color: #94a3b8;
}

.difficulty-tabs {
    display: flex;
    background-color: var(--secondary-color);
    border-radius: 10px;
    padding: 4px;
    margin-bottom: 20px;
}

.diff-tab {
    flex: 1;
    background: none;
    border: none;
    color: #94a3b8;
    padding: 10px;
    border-radius: 8px;
    font-family: var(--font-family);
    font-weight: 600;
    cursor: pointer;
    transition: all 0.2s;
}

.diff-tab.active {
    background-color: var(--primary-color);
    color: white;
}

.level-grid-container {
    flex: 1;
    overflow-y: auto;
    /* Custom Scrollbar */
    scrollbar-width: thin;
    scrollbar-color: var(--secondary-color) transparent;
}

.level-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(60px, 1fr));
    gap: 12px;
    padding-bottom: 20px;
}

.level-btn {
    aspect-ratio: 1;
    background-color: var(--secondary-color);
    border: 1px solid var(--grid-border);
    border-radius: 12px;
    color: white;
    font-size: 1.1rem;
    font-weight: 600;
    cursor: pointer;
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    transition: transform 0.1s, background-color 0.2s;
}

.level-btn:hover {
    background-color: #334155;
    transform: translateY(-2px);
}

.level-btn.completed {
    background-color: #064e3b;
    /* Dark Green */
    border-color: var(--success-color);
    color: var(--success-color);
}

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

.game-info {
    display: flex;
    flex-direction: column;
    align-items: center;
}

#difficulty-display {
    font-size: 0.8rem;
    color: #94a3b8;
    text-transform: uppercase;
    letter-spacing: 1px;
}

#level-display {
    font-weight: 700;
    font-size: 1.2rem;
}

.timer {
    font-family: monospace;
    font-size: 1.2rem;
    background-color: var(--secondary-color);
    padding: 5px 10px;
    border-radius: 6px;
}

.icon-btn {
    background: none;
    border: none;
    color: var(--text-color);
    cursor: pointer;
    padding: 8px;
    border-radius: 50%;
}

.icon-btn:hover {
    background-color: rgba(255, 255, 255, 0.1);
}

.game-board-container {
    display: flex;
    justify-content: center;
    margin-bottom: 20px;
}

.sudoku-board {
    display: grid;
    grid-template-columns: repeat(9, 1fr);
    width: 100%;
    aspect-ratio: 1;
    background-color: var(--grid-border);
    border: 2px solid var(--grid-border);
    gap: 1px;
    border-radius: 8px;
    overflow: hidden;
}

.cell {
    background-color: var(--bg-color);
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 1.5rem;
    font-weight: 500;
    cursor: pointer;
    user-select: none;
    position: relative;
    color: var(--text-color);
}

/* Thicker borders for 3x3 boxes */
.cell:nth-child(3n) {
    border-right: 2px solid var(--grid-border);
}

.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(--grid-border);
}

/* Cell States */
.cell.initial {
    font-weight: 700;
    color: #94a3b8;
    /* Dimmer color for initial numbers */
}

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

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

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

.cell.error {
    background-color: rgba(239, 68, 68, 0.2);
    color: var(--error-color);
}

.cell.highlighted-error {
    background-color: rgba(239, 68, 68, 0.4);
}


.controls-area {
    margin-top: auto;
    padding-bottom: 20px;
}

.action-buttons {
    display: flex;
    justify-content: space-around;
    margin-bottom: 20px;
}

.action-btn {
    background: none;
    border: none;
    color: var(--text-color);
    display: flex;
    flex-direction: column;
    align-items: center;
    font-family: var(--font-family);
    font-size: 0.9rem;
    gap: 5px;
    cursor: pointer;
    opacity: 0.8;
}

.action-btn:hover,
.action-btn.active {
    opacity: 1;
    color: var(--primary-color);
}

.action-btn.active svg {
    fill: rgba(59, 130, 246, 0.2);
}

.status-indicator {
    font-size: 0.6rem;
    background-color: #334155;
    padding: 2px 4px;
    border-radius: 4px;
}

.action-btn.active .status-indicator {
    background-color: var(--primary-color);
    color: white;
}

.numpad {
    display: grid;
    grid-template-columns: repeat(9, 1fr);
    gap: 5px;
}

.num-btn {
    aspect-ratio: 0.8;
    background-color: var(--secondary-color);
    border: none;
    border-radius: 8px;
    color: var(--primary-color);
    font-size: 1.5rem;
    font-family: var(--font-family);
    cursor: pointer;
    transition: background-color 0.1s;
}

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

/* Modal */
.modal {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background-color: rgba(0, 0, 0, 0.8);
    display: flex;
    align-items: center;
    justify-content: center;
    z-index: 100;
    opacity: 1;
    transition: opacity 0.3s;
}

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

.modal-content {
    background-color: var(--secondary-color);
    padding: 40px;
    border-radius: 20px;
    text-align: center;
    width: 90%;
    max-width: 400px;
    transform: translateY(0);
    transition: transform 0.3s;
}

.modal.hidden .modal-content {
    transform: translateY(50px);
}

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

.stars {
    font-size: 3rem;
    color: #fbbf24;
    margin-bottom: 20px;
}

.modal-actions {
    display: flex;
    flex-direction: column;
    gap: 10px;
}

.primary-btn,
.secondary-btn {
    padding: 15px;
    border-radius: 12px;
    border: none;
    font-size: 1.1rem;
    font-weight: 600;
    cursor: pointer;
    font-family: var(--font-family);
}

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

.secondary-btn {
    background-color: transparent;
    border: 2px solid var(--grid-border);
    color: #94a3b8;
}

/* Notes in cell */
.cell-notes {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    width: 100%;
    height: 100%;
    pointer-events: none;
}

.note-num {
    font-size: 0.6rem;
    color: #94a3b8;
    display: flex;
    align-items: center;
    justify-content: center;
}

@media (max-width: 400px) {
    .cell {
        font-size: 1.2rem;
    }

    .num-btn {
        font-size: 1.2rem;
    }
}

/* Number Popup Bubble */
.number-popup {
    position: absolute;
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    gap: 8px;
    background-color: var(--secondary-color);
    padding: 10px;
    border-radius: 12px;
    box-shadow: 0 10px 25px rgba(0, 0, 0, 0.5);
    z-index: 50;
    transform: translate(-50%, -50%) scale(1);
    transition: opacity 0.2s, transform 0.2s;
    pointer-events: auto;
    border: 1px solid var(--grid-border);
}

.number-popup.hidden {
    opacity: 0;
    pointer-events: none;
    transform: translate(-50%, -50%) scale(0.8);
}

.popup-btn {
    width: 40px;
    height: 40px;
    border-radius: 50%;
    border: 1px solid var(--grid-border);
    background-color: var(--bg-color);
    color: var(--text-color);
    font-size: 1.2rem;
    font-weight: 600;
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    transition: all 0.1s;
}

.popup-btn:hover {
    background-color: var(--primary-color);
    color: white;
    transform: scale(1.1);
}

.popup-btn.action {
    background-color: rgba(239, 68, 68, 0.1);
    color: var(--error-color);
    grid-column: span 3;
    /* Make clear button span full width or just put it in grid */
    width: 100%;
    border-radius: 8px;
    height: 30px;
    font-size: 1rem;
}

.popup-btn.action:hover {
    background-color: var(--error-color);
    color: white;
}

/* Message Modal */
.message-modal {
    position: fixed;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    background-color: var(--secondary-color);
    padding: 20px 30px;
    border-radius: 12px;
    box-shadow: 0 10px 25px rgba(0, 0, 0, 0.5);
    z-index: 200;
    text-align: center;
    border: 1px solid var(--grid-border);
    min-width: 250px;
    transition: opacity 0.2s, transform 0.2s;
}

.message-modal.hidden {
    opacity: 0;
    pointer-events: none;
    transform: translate(-50%, -40%);
}

.message-content p {
    font-size: 1.1rem;
    margin-bottom: 20px;
    color: var(--text-color);
}

.small-btn {
    padding: 8px 20px;
    font-size: 1rem;
}