@media (max-width: 960px) {
    .main {
        background-color: green;
    }
    .sidebar-first {
        background-color: red;
    }
    .sidebar-second {
        background-color: blue;
    }
}
@media (max-width: 768px) {
    .columns {
        flex-wrap: wrap;
    }
    .sidebar-first,
    .main {
        width: 50%;
    }
    .sidebar-second {
        width: 100%;
    }
}
@media (max-width: 540px) {
    .columns {
        flex-direction: column;
    }
    .sidebar-first,
    .main,
    .sidebar-second {
        width: 100%;
    }
}
@media (max-width: 480px) {
    .main {
        order: 1;
    }
    .sidebar-first {
        order: 2;
    }
    .sidebar-second {
        order: 3;
    }
}