[CSSチュートリアル】css3 背景画像 半透明 コンテンツ 不透明 方法例

.login_box::before{
            content:"";
           
            background-image:url(images/one.jpg);
            opacity:0.5;//transparency setting
            z-index:-1;
            background-size:500px 300px;
            width:500px; 
            height:300px;
            position:absolute;
            // Be sure to set position:absolute, so that the z-index can be set and the background can be placed at the next level of the content
            top:0px;
            left:0px;
            border-radius:40px;
        }
         .login_box{
            position:fixed;
            left:50%;
            top:200px;
            width:500px;
            height:300px;
            margin-left:-250px;
            border-radius:40px;
            box-shadow: 10px 10px 5px #888;
            border:1px solid #666;
 
            text-align:center;
        }

<div class="bg">
    <div class="content">
    Content
    </div>
</div>