body {
    font-family: 'Poppins', sans-serif;
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: flex-start;
    min-height: 100vh;
    margin: 0;
    background-color: #f0f0f0;
    color: #333;
}
h1 {
    margin-top: 20px;
    margin-bottom: 10px;
    color: #2c3e50;
}
#difficulty-container {
    display: flex;
    margin-bottom: 20px;
}
#difficulty-container button {
    padding: 10px 15px;
    margin: 0 5px;
    border: none;
    border-radius: 5px;
    background-color: #ecf0f1;
    color: #2c3e50;
    cursor: pointer;
    font-family: 'Poppins', sans-serif;
    transition: background-color 0.3s, color 0.3s;
}
#difficulty-container button:hover {
    background-color: #3498db;
    color: white;
}
#difficulty-container button.selected {
    background-color: #3498db;
    color: white;
}
#sudoku-board {
    border-collapse: collapse;
    box-shadow: 0 4px 8px rgba(0, 0, 0, 0.1);
    margin-bottom: 20px;
    max-width: 90vw;
}
#sudoku-board tr {
    border: none;
}
#sudoku-board td {
    border: none;
    padding: 0;
}
#sudoku-board input {
    width: 8vw;
    height: 8vw;
    text-align: center;
    font-size: 3vw;
    border: 1px solid #bdc3c7;
    border-radius: 0;
    outline: none;
    font-family: 'Poppins', sans-serif;
    background-color: white;
    transition: background-color 0.2s;
    padding: 0;
    box-sizing: border-box;
    max-width: 50px;
    max-height: 50px;
}
#sudoku-board input:disabled {
    background-color: #ecf0f1;
    color: #2c3e50;
}
#sudoku-board input:focus {
    background-color: #e0f7fa;
    outline: none;
}
#sudoku-board tr:nth-child(3n) td {
    border-bottom: 2px solid #3498db;
}
#sudoku-board td:nth-child(3n) {
    border-right: 2px solid #3498db;
}
#timer {
    font-size: 1.5em;
    margin-bottom: 20px;
    color: #7f8c8d;
}
#reset-button {
    padding: 10px 20px;
    border: none;
    border-radius: 5px;
    background-color: #ff6b6b;
    color: white;
    cursor: pointer;
    font-family: 'Poppins', sans-serif;
    transition: background-color 0.3s;
    font-size: 1em;
    margin-bottom: 20px;
}
#reset-button:hover {
    background-color: #e74c3c;
}
#game-over-modal {
    display: none;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    background-color: rgba(0, 0, 0, 0.8);
    color: white;
    padding: 20px;
    border-radius: 10px;
    font-size: 1.2em;
    position: fixed;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    z-index: 10;
    width: 90%;
    max-width: 300px;
}
#game-over-modal button {
    margin-top: 10px;
    padding: 10px 20px;
    border: none;
    border-radius: 5px;
    background-color: #2ecc71;
    color: white;
    cursor: pointer;
    font-family: 'Poppins', sans-serif;
    transition: background-color 0.3s;
    font-size: 0.9em;
}
#game-over-modal button:hover {
    background-color: #27ae60;
}
@media (max-width: 400px) {
    #sudoku-board input {
        font-size: 2.5vw;
    }
    #timer {
        font-size: 1.2em;
    }
    #game-over-modal {
        font-size: 1em;
        padding: 15px;
    }
}