body {
    font-family: Arial, sans-serif;
    margin: 0;
    padding: 0;
    background: #f7f7f7;
}
nav {
    background: #333;
    padding: 0.5em 0;
}
nav ul {
    list-style: none;
    margin: 0;
    padding: 0;
    display: flex;
    justify-content: center;
    gap: 2em;
}
nav li {
    display: inline;
}
nav a {
    color: #fff;
    text-decoration: none;
    font-size: 1.1em;
    padding: 0.5em 1em;
    border-radius: 4px;
    transition: background 0.2s;
}
nav a:hover {
    background: #555;
}
main {
    padding: 2em;
    text-align: center;
}

button.doublePendulum {
    font-family: Arial, sans-serif;
    border: black 2px solid;
    color: black;
    padding: 10px 20px;
    text-align: center;
    text-decoration: none;
    display: inline-block;
    font-size: 16px;
    margin: 4px 20px;
    cursor: pointer;
}