.wrapper {
    display: flex;
    flex-direction: column;
    height: 100vh;
}
header,
footer {
    background-color: #666;
}
footer {
    height: 50px;
}
ol {
    display: flex;
}
a {
    text-decoration: none;
    color: black;
    padding: 1rem;
    display: block;
}
li:first-child a {
    border-right: black solid 1px;
}
.content {
    background-color: #ccc;
    flex: 1;
}
.columns {
    display: flex;
    height: 100%;
}
.main {
    flex: 1;
    background-color: #ccc;
}
.sidebar {
    width: 20%;
    background-color: #999;
}

@media (max-width: 640px) {
    .columns {
        flex-direction: column;
    }
    .sidebar {
        width: 100%;
        height: 5rem;
    }
}