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

body {
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
    min-height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    padding: 20px;
}

.container {
    background: white;
    border-radius: 20px;
    padding: 30px;
    box-shadow: 0 20px 60px rgba(0, 0, 0, 0.3);
    max-width: 650px;
    width: 100%;
}

h1 {
    text-align: center;
    color: #333;
    margin-bottom: 30px;
    font-size: 2.5em;
}

.controls {
    display: flex;
    gap: 10px;
    margin-bottom: 20px;
    justify-content: center;
    flex-wrap: wrap;
}

.controls select,
.controls button {
    padding: 10px 20px;
    border: none;
    border-radius: 8px;
    font-size: 16px;
    cursor: pointer;
    transition: all 0.3s;
}

.controls select {
    background: #f0f0f0;
    color: #333;
}

.controls button {
    background: #667eea;
    color: white;
}

.controls button:hover {
    background: #5a67d8;
    transform: translateY(-2px);
    box-shadow: 0 5px 15px rgba(102, 126, 234, 0.4);
}

.message {
    text-align: center;
    padding: 10px;
    margin-bottom: 20px;
    border-radius: 8px;
    font-weight: bold;
    min-height: 40px;
    display: flex;
    align-items: center;
    justify-content: center;
    transition: all 0.3s;
}

.message.success {
    background: #d4edda;
    color: #155724;
    border: 1px solid #c3e6cb;
}

.message.error {
    background: #f8d7da;
    color: #721c24;
    border: 1px solid #f5c6cb;
}

.message.info {
    background: #d1ecf1;
    color: #0c5460;
    border: 1px solid #bee5eb;
}

.sudoku-board {
    display: grid;
    grid-template-columns: repeat(9, 1fr);
    gap: 1px;
    background: #333;
    padding: 3px;
    border-radius: 10px;
    margin-bottom: 20px;
    aspect-ratio: 1;
    max-width: 500px;
    margin-left: auto;
    margin-right: auto;
}

.cell {
    background: white;
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 20px;
    font-weight: bold;
    cursor: pointer;
    transition: all 0.2s;
    aspect-ratio: 1;
    position: relative;
    user-select: none;
}

.cell-input {
    width: 100%;
    height: 100%;
    border: none;
    text-align: center;
    font-size: 20px;
    font-weight: bold;
    background: transparent;
    color: #2563eb;
    outline: none;
    cursor: pointer;
}

.cell-input:focus {
    background: #e6f3ff;
}

.cell.fixed {
    background: #f5f5f5;
    color: #333;
    cursor: not-allowed;
}

.cell.fixed .cell-input {
    color: #333;
    cursor: not-allowed;
}

.cell.error .cell-input {
    background: #ffe6e6;
    color: #cc0000;
}

.cell.hint {
    animation: pulse 1s ease-in-out;
}

.cell.hint .cell-input {
    background: #fffacd;
    color: #f59e0b;
}

/* Thick borders for 3x3 boxes */
.cell:nth-child(3n) {
    border-right: 3px solid #333;
}

.cell:nth-child(n+19):nth-child(-n+27),
.cell:nth-child(n+46):nth-child(-n+54) {
    border-bottom: 3px solid #333;
}

.instructions {
    background: #f0f4f8;
    border-radius: 10px;
    padding: 15px;
    margin-bottom: 20px;
    border-left: 4px solid #667eea;
}

.instructions p {
    margin-bottom: 10px;
    color: #333;
}

.instructions ul {
    margin-left: 20px;
    color: #555;
}

.instructions li {
    margin-bottom: 5px;
}

@keyframes pulse {
    0% { transform: scale(1); }
    50% { transform: scale(1.05); }
    100% { transform: scale(1); }
}

/* Responsive Design */
@media (max-width: 600px) {
    .container {
        padding: 20px;
    }

    h1 {
        font-size: 2em;
    }

    .cell-input {
        font-size: 16px;
    }

    .controls {
        flex-direction: column;
    }

    .controls select,
    .controls button {
        width: 100%;
    }
}