.bg{
    background: blueviolet;
    width: 100%;
    height: 100vh;
    background-size: 200%;
}
.form-container{
    position: absolute;
    margin-top: 10vh;
    background: #fff;
    padding: 30px;
    border-radius: 10px;
    box-shadow: 0px 0px 10px 0px #000;
}

@media only screen and (max-width:670px){
    .bg{
        background-size: 300%;
    }
}