ざっくりとTOPへ戻るボタンを追加しました。
#page_top{
width: 50px;
height: 50px;
position: fixed;
right: 0;
bottom: 0;
background: #3fefee;
opacity: 0.6;
}
#page_top a{
position: relative;
display: block;
width: 50px;
height: 50px;
text-decoration: none;
}
#page_top a::before{
font-family: FontAwesome;
content: '\f062';
font-size: 25px;
color: #fff;
position: absolute;
width: 25px;
height: 25px;
top: 0;
bottom: 0;
right: 0;
left: 0;
margin: auto;
text-align: center;
}
<div id="page_top"><a href="#"></a></div>