body {
    background-image: url("bg.jpg")
}

nav {
    background-color: #333;
    overflow: hidden;
}

nav a {
    float: left;
    color: #f2f2f2;
    text-align: center;
    padding: 14px 16px;
    text-decoration: none;
    font-size: 17px;
    background-color: black;
    background-size: 20px;
    width: 200px;
}

nav a:hover {
    background-color: #ddd;
    color: black;

}

nav a.active {
 background-color: #4CAF50;
 color: white;
}

{
    display: block;
    margin-left: auto;
    margin-right: auto;
    width: 50%;
    height: 20px;
}

.center {
    display: block;
    margin-left: auto;
    margin-right: auto;
    width: 50%;
}

logo {
    display: block;
    margin-left: auto;
    margin-right: auto;
    width: 20%;
    height: 20%;
}

main {
    display: block;
    margin-left: auto;
    margin-right: auto;
    width: 50%;
}