.header{
    background: linear-gradient(180deg, #000000 56%, #ad0000 100%);
    height: 100px;
    width: 100%;
    position: fixed;
    z-index: 1;
    top: 0;
}

.mainback{
    z-index: 0;
    position: fixed;   
    top: 0; 
    width: 100%;
}

.mainbox{
    background-color: white;
    margin-top: 400px;
    width: 100%;
}