div, nav, header, footer {
    border: 1px solid black;
}

body {
    display: flex;
    flex-direction: column;
    align-items: center;
}

#banner {
    display: flex;
    justify-content: center;
    width: 75%;
}

 #main-header {
    display: flex;
    width: 75%;
} 

#main-header nav {
    width: 100%;
}

#main-header ul {
    display: flex;
    list-style-type: none;
    padding: 0;
    justify-content: space-evenly;
}