SOURCE

<div class="con">
    <div class="box header">
        <div
            style="height: 50px"
        ></div>
    </div>
    <div class="box body"></div>
    <div class="box header">
        <div
            style="height: 50px"
        ></div>
    </div>
</div>

.con{
    height: 500px;
    border: 1px solid #000;
    display: flex;
    flex-direction: column;
}

.header{
    border-bottom: 1px solid yellowgreen
}
.body{
    flex: auto;
    background-color: #123456;
}