body {
    background-color: #121212;
    color: #ffffff;
    font-family: Arial, sans-serif;
    margin: 0;
    padding: 20px;
    display: flex;
    justify-content: center;
    align-items: center;
}
.quiz-container {
    background-color: #1e1e1e;
    padding: 20px;
    border-radius: 8px;
    box-shadow: 0 0 10px rgba(0, 0, 0, 0.5);
    width: 80%;
}
h1 {
    text-align: center;
    color: #4caf50;
}
#question {
    font-size: 18px;
    margin-bottom: 20px;
}
#answer {
    width: 100%;
    padding: 10px;
    margin-bottom: 10px;
    background-color: #333;
    color: #fff;
    border: 1px solid #555;
    border-radius: 4px;
}
#submit, #next {
    padding: 10px 20px;
    background-color: #4caf50;
    color: white;
    border: none;
    border-radius: 4px;
    cursor: pointer;
    margin-right: 10px;
}
#submit:disabled, #next:disabled {
    background-color: #555;
    cursor: not-allowed;
}
#result {
    margin-top: 20px;
    font-size: 16px;
    color: #ffeb3b;
}
#progress {
    text-align: center;
    margin-bottom: 20px;
    font-size: 16px;
}
.navigation {
    display: flex;
    flex-wrap: wrap;
    justify-content: center;
    margin-top: 20px;
}
.nav-btn {
    width: 37.5px;
    height: 37.5px;
    margin: 5px;
    border: none;
    border-radius: 50%;
    color: white;
    cursor: pointer;
    font-size: 14px;
}
.nav-btn.unanswered {
    background-color: #555;
    cursor: not-allowed;
}
.nav-btn.correct {
    background-color: #4caf50; /* Xanh lá cây */
}
.nav-btn.incorrect {
    background-color: #f44336; /* Đỏ */
}
.nav-btn.current {
    background-color: #2196f3; /* Xanh dương */
}