*{
margin:5px;
padding:0;
box-sizing:border-box;
}
.body{
    padding:20px;
    
}


.box {
   width:590px;
   height:auto;
   background-color:rgb(255, 255, 255);
/**//**/   position:relative;
   border-radius:15px;
}
    .box::before {
        content: '';
        background: linear-gradient(to top,black,brown);
        /**/ position: absolute;
        text-align: left;
        
        top: 0;
        left: 0;
        right: 0;
        bottom: 0;
        z-index: -1;
        margin: -5px;
        border-radius: 15px;
    }