style.css

html,body{
    font-family: Arial, Helvetica, sans-serif;
    margin: 0px;
    padding: 0px;
}

header{
    padding: 16px;
    background-color: whitesmoke;
}
    header nav{
        background-color: snow;
        border: 1px solid lightgrey;
        border-radius: 8px;
        padding: 16px;
        display: flex;
        justify-content: space-between;
    }
        header nav a{
            text-decoration: none;
        }

main{
    padding: 16px;
    display: flex;
    flex-wrap: wrap;
    align-items: center;
    justify-content: center;
    column-gap: 16px;
    row-gap: 16px;
}
    .notebook{
        text-decoration: none;
        color: white !important;
        font-size: 1.5rem;
        background-color: black;
        border-radius: 8px;
    }
        .notebook>div{
            height: 200px;
            width: 200px;
            padding: 8px;
            box-sizing: border-box;
            display: flex;
            align-items: center;
            justify-content: center;
            text-align: center;
        }

header>*, main{
    margin-left: auto;
    margin-right: auto;
    width: 100%;
    max-width: 800px;
}