pages/src/lib/content-sidebar.svelte

<div class="content">
    <div class="side">
        <slot name="side-left" />
    </div>
    <div class="main">
        <slot name="main" />
    </div>
    <div class="side">
        <slot name="side-right" />
    </div>
</div>

<style>
    .content {
        max-width: 2000px;
        margin: 0 auto 228px;
        display: flex;
        flex-direction: row;
    }
    .side {
        min-width: 400px;
    }
</style>