[RSS Feed/News] 4 boxes in row on desktop, how to column them in mobile ?

[XenForo]

This is the CSS code, what should I add to it so that my 4 boxes are vertically aligned on small screens?
Less:

.box-container {
    display: flex;
    }
    
    .column {
        display: flex;
        flex: 1 1 250px;
        font-size: 21px;
        font-weight: 400;
        margin-left: 0;
        padding: 8px;
        text-align: center;
    }

]

Many thanks.

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