body {
    font-family: Georgia, serif;
    max-width: 900px;
    margin: auto;
    padding: 20px;
    line-height: 1.7;
    background: #f8f8f8;
    color: #222;
}
header {
    background: #2c3e50;
    color: white;
    padding: 20px;
    border-radius: 8px;
}
nav a {
    margin-right: 15px;
    color: #dfefff;
    text-decoration: none;
}
article {
    background: white;
    padding: 25px;
    margin-top: 20px;
    border-radius: 8px;
}
h1, h2 {
    color: #1a3d5c;
}
.footer {
    margin-top: 30px;
    font-size: 0.9em;
    color: #666;
}
.problem {
    background: #eef5ff;
    padding: 10px;
    border-left: 4px solid #5b8bd9;
    margin: 10px 0;
}