Selamlar css de body taginde sorun var şunu deneyin.
body {
font-family: 'Ropa Sans', Arial, Helvetica, sans-serif;
background: url('https://2.bp.blogspot.com/-FDqE1n7scyk/WOOpJ_8SCBI/AAAAAAAAHDU/CHDZ4ea0jfoIEmHaDBCKcFPYsWWQgsHwgCLcB/s1600/cats11.jpg');
background-repeat: no-repeat;
background-size: cover;
background-color: #fffff5;
}

Tercihe göre
background-position-x: center;
da eklenebilir