body {
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    background-color: #f4f4f9;
    color: #333;
    display: flex;
    flex-direction: column;
    align-items: center;
    margin: 0;
    padding: 20px;
    min-height: 100vh;
}

.game-header {
    width: 100%;
    max-width: 500px;
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 20px;
}

.back-btn {
    text-decoration: none;
    color: #555;
    font-weight: bold;
    padding: 5px 10px;
    background: #e0e0e0;
    border-radius: 4px;
}

.back-btn:hover {
    background: #d0d0d0;
}

h1 {
    margin: 0;
    color: #2c3e50;
}

.game-container {
    background: white;
    padding: 20px;
    border-radius: 12px;
    box-shadow: 0 4px 15px rgba(0,0,0,0.1);
    display: flex;
    flex-direction: column;
    align-items: center;
    width: 100%;
    max-width: 500px;
}

.controls-top {
    margin-bottom: 20px;
    display: flex;
    gap: 10px;
    align-items: center;
    width: 100%;
    justify-content: center;
}

select {
    padding: 5px;
    border-radius: 4px;
    border: 1px solid #ccc;
}

button {
    padding: 6px 12px;
    border-radius: 4px;
    border: none;
    cursor: pointer;
    font-weight: bold;
    background-color: #3498db;
    color: white;
    transition: background 0.2s, transform 0.1s;
}

button:active {
    transform: translateY(1px);
}

button:hover {
    background-color: #2980b9;
}

button#reset-btn {
    background-color: #e74c3c;
}
button#reset-btn:hover { background-color: #c0392b; }

button#check-btn {
    background-color: #27ae60;
}
button#check-btn:hover { background-color: #219150; }

.sudoku-board {
    display: grid;
    grid-template-columns: repeat(9, 1fr);
    width: 100%;
    aspect-ratio: 1;
    background-color: #333;
    border: 2px solid #333;
    gap: 1px;
    margin-bottom: 20px;
}

.cell {
    background-color: white;
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: clamp(1rem, 4vw, 1.8rem);
    cursor: pointer;
    user-select: none;
    position: relative;
    color: #2c3e50;
}

/* Notes Grid */
.cell .notes-grid {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    grid-template-rows: repeat(3, 1fr);
    width: 100%;
    height: 100%;
    position: absolute;
    top: 0;
    left: 0;
    pointer-events: none;
}

.cell .note-num {
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 0.35em;
    color: #7f8c8d;
    font-weight: bold;
    line-height: 1;
}

/* Borders */
.cell.b-right { border-right: 2px solid #333; }
.cell.b-bottom { border-bottom: 2px solid #333; }

/* States */
.cell.fixed {
    background-color: #f0f0f0;
    font-weight: bold;
    color: #2c3e50;
}

.cell.player-input {
    color: #2980b9;
}

.cell.selected {
    background-color: #bbdefb; /* Blue selection */
}

.cell.related {
    background-color: #e8f6f3; /* Crosshair subtle highlight */
}

/* If related AND fixed, maybe blend? */
.cell.fixed.related {
    background-color: #e0e6e6;
}

/* If selected AND fixed */
.cell.fixed.selected {
    background-color: #b0bec5;
}

.cell.highlight-match {
    background-color: #d6eaf8; /* Lighter blue for matching numbers */
}

.cell.selected.highlight-match {
    background-color: #aed6f1; /* Darker match if selected */
}

.cell.error {
    color: #e74c3c;
    background-color: #fadbd8 !important;
}

/* Bottom Controls */
.controls-bottom {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 15px;
    width: 100%;
}

.tools {
    display: flex;
    gap: 10px;
    width: 100%;
    justify-content: center;
}

.tool-btn {
    flex: 1;
    padding: 12px;
    background: #ecf0f1;
    color: #34495e;
    font-size: 0.9rem;
    border: 1px solid #bdc3c7;
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 5px;
}

.tool-btn:hover { background: #dfe6e9; }

.tool-btn#undo-btn { }
.tool-btn#note-btn.active {
    background: #3498db;
    color: white;
    border-color: #2980b9;
}

.tool-btn#erase-btn { color: #c0392b; }

.numpad {
    display: grid;
    grid-template-columns: repeat(9, 1fr);
    gap: 5px;
    width: 100%;
}

.num-btn {
    aspect-ratio: 1;
    font-size: 1.2rem;
    background: #f8f9fa;
    border: 1px solid #dee2e6;
    color: #333;
    display: flex;
    align-items: center;
    justify-content: center;
    padding: 0;
}

.num-btn:hover { background: #e9ecef; }

.num-btn.selected-number {
    background-color: #3498db;
    color: white;
    border-color: #2980b9;
    box-shadow: inset 0 0 5px rgba(0,0,0,0.2);
}

.actions {
    display: flex;
    gap: 10px;
    width: 100%;
    justify-content: center;
}

.status-message {
    min-height: 24px;
    font-weight: bold;
    color: #27ae60;
    text-align: center;
}

@media (max-width: 500px) {
    .game-container { padding: 10px; }

    .numpad {
        /* 2 rows for easier touch targets */
        grid-template-columns: repeat(5, 1fr);
    }
    /* Make the 9th item (last) span full width or center?
       With repeat(5), 9 items fill 2 rows (5 + 4).
       The last 4 will leave a gap. */

    .cell {
        font-size: clamp(1rem, 4vw, 1.5rem);
    }
}