Theming: Set BG-color of `backdrop`ped, selection on `GtkLabel`s

I’d like to set a specific color for selections on labels, that are backdropped. How can I do that?

The natural selection:backdrop {background: lightgray;} does not have any effect.

Tests

I tested with the About dialog of gtk3-widget-factory, page 2.

image

This worked:
label:backdrop selection {background: lightgray;}

However, as the general background color is set via selection {background: blue;}, I think this could be considered a bug?

FYI: I created an issue here: https://gitlab.gnome.org/GNOME/gtk/-/issues/4303