Bulma class: .is-narrow-mobile

    <div class="columns">
        <div class="column is-narrow-mobile">
            <div class="box" style="width: 200px;">
                <p class="title is-5">Narrow column on mobile</p>
                <p class="subtitle">This column is only 200px wide.</p>
            </div>
        </div>
        <div class="column">
            <div class="box">
                <p class="title is-5">Flexible column</p>
                <p class="subtitle">This column will take up the remaining space available.</p>
            </div>
        </div>
    </div>