The GtkList has items of the class ../GtkListItem. These list items will draw the background if they are selected using a flat_box. The state is of course SELECTED, detail string is "listitem" and the shadow is ETECHED_OUT. The default background is base[NORMAL] (each of them has their own X window, which means no transparency, etc.)
You can work around this problem with a match similar to the following one:
widget_class "*<GtkListItem>*" style "list-item"