Theming: Backdrop coloring of selected GtkListBox rows with padding

Problem
I can’t figure out how to properly set the backdrop color of selected rows with padding of a GtkListBox.

This is the CSS I’m using (which doesn’t work)

row { padding: 5px; }

/* set selection colors */
list :selected { background: lightblue; }
row  :selected { background: lightblue; } /* seems to have no effect */

/* set backdrop colors */
row  :selected :backdrop { background: lightgray; } /* seems to have no effect */
list :selected :backdrop { background: lightgray; }

(Can be used in the inspector)

This is the resulting outcome (from gtk3-widget-factory, page 3):
image

As you can see, the row's light blue selected color remains in the padding area. I.e. row :selected :backdrop does not show effect.

In case this is a bug of GTK, I’m also very interested in that information as I can then plan accordingly.

It’s row:selected:backdrop, without the spaces.