>>198
style.cssの最後にこれを追加してみて

body.custom-background {
background-attachment: fixed !important;
}

これで無理なら変わりに以下を追加

body.custom-background {
background-position: center center !important;
background-size: cover !important;
background-repeat: no-repeat !important;
background-attachment: fixed !important;
}