GUI: Use `TabContainer` material for internal `TabBar`

scene/gui/tab_container.cpp

@@ -1167,6 +1167,7 @@ TabContainer::TabContainer() {
 	tab_bar = memnew(TabBar);
 	SET_DRAG_FORWARDING_GCDU(tab_bar, TabContainer);
 	add_child(tab_bar, false, INTERNAL_MODE_FRONT);
+	tab_bar->set_use_parent_material(true);
 	tab_bar->set_anchors_and_offsets_preset(Control::PRESET_TOP_WIDE);
 	tab_bar->connect("tab_changed", callable_mp(this, &TabContainer::_on_tab_changed));
 	tab_bar->connect("tab_clicked", callable_mp(this, &TabContainer::_on_tab_clicked));