body {

justify-content: center;
align-items: center;
min-height: 100vh;
background : url('https://i.ibb.co/3F36pnW/css.png') no-repeat;
background-position: top;
}

header {
justify-content: center;
align-items: center;
min-height: 100vh;
background : url('https://i.ibb.co/H7krCLp/css2.jpg') no-repeat;
background-size: cover;
background-position: center;
}