首尾DIV固定，中間DIV能夠自適應高度

• 範例

.custom-font{
             top:50%;
             transform: translateY(-50%);
             font-size: 30px;
         }
.custom-content{
             top:70px;
             bottom:70px;
             left: 0px;
         }
.custom-inner{
            //內容CSS在此調整
         }
.custom-shadow{
             box-shadow: 0 -.125rem .25rem rgba(0,0,0,.075)!important;
         }

<div class="container">
        <div class="fixed-top border-bottom text-center shadow-sm" style="height: 70px;">
            <span class="custom-font position-relative d-inline-block">TOP NAV</span>
        </div>
        <div class="custom-content position-absolute overflow-auto bg-light w-100">
            <div class="custom-inner p-3">
                <p>內容</p>
                <p>內容</p>
                <p>內容</p>
                <p>內容</p>
                <p>內容</p>
                <p>內容</p>
                <p>內容</p>
                <p>內容</p>
                <p>內容</p> 
                <p>內容</p>
                <p>內容</p>
                <p>內容</p>
                <p>內容</p>
                <p>內容</p>
                <p>內容</p>
                <p>內容</p>
                <p>內容</p>
                <p>內容</p>
                <p>內容</p>
                <p>內容</p>
                <p>內容</p>
            </div>
        </div>
        <div class="fixed-bottom border-top text-center custom-shadow" style="height: 70px;">
            <span class="custom-font position-relative d-inline-block">BOTTOM NAV</span>
        </div>
    </div>

• 結果