I could not get this to obey using .SG_card for a background. I used Chrome developer tools and had to in custom.css:
.ipsBox:not( .ipsBox_transparent ) .ipsAreaBackground_reset, .ipsBox .ipsTabs_panel, .ipsfocusBox .ipsTabs_panel, .ipsfocus_reset.ipsBox, .ipsDialog_content .ipsBox, .ipsDialog > div .ipsDialog_content .ipsBox, .ipsfocusBox .ipsComment.cPost:not(.ipsModerated):not(.ipsComment_selected), .ipsBox.ipsfocus_reset:not(.ipsBox_transparent):not(.ipsModerated), .ipsBox:not(.ipsfocus_reset) .ipsfocusBox {
/* background: none; */
background-color: #ffffff;
}
This fixed all of my issues although I don't know if all of these elements are needed.