### Godot version

4.0.dev (cd031fd31a8fdcdae1e93d1b20caf93230f0ac59)

### System information

Windows 10

### Issue description

When using the generic SplitContainer (not the H/V class variations), with the default theme, the grabber icon is missing, both when the vertical bool is set to true or false:

![image](https://user-images.githubusercontent.com/5117197/213923999-2762f456-5bc1-4541-afa2-f39de051b4ad.png)


### Steps to reproduce

- Create SplitContainer (not HSplitContainer or VSplitContainer)
- Give it two children so it renders properly
- Hover grabber in-game

### Minimal reproduction project

[split_container_icon.zip](https://github.com/godotengine/godot/files/10474735/split_container_icon.zip)

} else {
				dragging = false;
				queue_redraw();
			}
		}
	}
	Ref<InputEventMouseMotion> mm = p_event;
	if (mm.is_valid()) {
		if (!dragging) {
			return;
		}
		Vector2i in_parent_pos = get_transform().xform(mm->get_position());
		if (!sc->vertical && is_layout_rtl()) {
			sc->split_offset = drag_ofs - (in_parent_pos.x - drag_from);
		} else {
			sc->split_offset = drag_ofs + ((sc->vertical ? in_parent_pos.y : in_parent_pos.x) - drag_from);
		}
		sc->_compute_middle_sep(true);
		sc->queue_sort();
		sc->emit_signal(SNAME("dragged"), sc->get_split_offset());
	}
}
Control::CursorShape SplitContainerDragger::get_cursor_shape(const Point2 &p_pos) const {
	SplitContainer *sc = Object::cast_to<SplitContainer>(get_parent());
	if (!sc->collapsed && sc->dragger_visibility == SplitContainer::DRAGGER_VISIBLE) {
		return (sc->vertical ? CURSOR_VSPLIT : CURSOR_HSPLIT);
	}
	return Control::get_cursor_shape(p_pos);
}
void SplitContainerDragger::_notification(int p_what) {