#container {
    border: 1px solid rgb(32, 76, 221);
}

div {
    border: 1px solid black;
}

#container > *:not(#middle-flow), #middleflow >* {
    /* wybór wszystkich pierwszych elementów */
    
    min-height: 100px;
    
    display: flex;
    justify-content: center;
    align-items: center;
    
    }

#middle-flow {
    display: flex;
    margin: 10px 0;
    flex-direction: row;
}

header {
    background-color: green;
}
nav {
    flex: 1;
    background-color: blue;
}
 
article {
    flex: 7;
    background-color: yellow;

    padding: 10px;
    margin: 0 10px;
}
 
aside {
    flex: 1;
    background-color: teal;
}
 
footer {
    background-color: red;
}