site/_src/_assets/styles/_page-404.scss

.page-404 {
    .header {
        background-image: url('/assets/img/fail-cat.gif');
        min-height: 420px;

        @media ($screen-md) {
            min-height: 100vh;
        }
    }
    .content--page {
        text-align: center;
    }
}