That’s because you are looking at the gnome-terminal inspector and as far as I can tell from your screenshot you are not using header bars in gnome-terminal, so you the decorations are not drawn by gnome-terminal but by mutter. Since 44 mutter uses gtk4 for the window decorations and in gtk4 the decorations use the window selector.
Changing the background of the window this way likely won’t work, because the titlebar is drawn on top of this, so you would need to change the background of the corresponding selector.
Or just use header bars in gnome-terminal: gsettings set org.gnome.Terminal.Legacy.Settings headerbar true. This should be the default, but maybe you changed it in the past. With header bars, everything is drawn by gnome-terminal and you can modify it using the gtk3 gtk.css and everything should show up in the inspector.