### 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)

ClassDB::bind_method(D_METHOD("set_split_offset", "offset"), &SplitContainer::set_split_offset);
	ClassDB::bind_method(D_METHOD("get_split_offset"), &SplitContainer::get_split_offset);
	ClassDB::bind_method(D_METHOD("clamp_split_offset"), &SplitContainer::clamp_split_offset);
	ClassDB::bind_method(D_METHOD("set_collapsed", "collapsed"), &SplitContainer::set_collapsed);
	ClassDB::bind_method(D_METHOD("is_collapsed"), &SplitContainer::is_collapsed);
	ClassDB::bind_method(D_METHOD("set_dragger_visibility", "mode"), &SplitContainer::set_dragger_visibility);
	ClassDB::bind_method(D_METHOD("get_dragger_visibility"), &SplitContainer::get_dragger_visibility);
	ClassDB::bind_method(D_METHOD("set_vertical", "vertical"), &SplitContainer::set_vertical);
	ClassDB::bind_method(D_METHOD("is_vertical"), &SplitContainer::is_vertical);
	ADD_SIGNAL(MethodInfo("dragged", PropertyInfo(Variant::INT, "offset")));
	ADD_PROPERTY(PropertyInfo(Variant::INT, "split_offset", PROPERTY_HINT_NONE, "suffix:px"), "set_split_offset", "get_split_offset");
	ADD_PROPERTY(PropertyInfo(Variant::BOOL, "collapsed"), "set_collapsed", "is_collapsed");
	ADD_PROPERTY(PropertyInfo(Variant::INT, "dragger_visibility", PROPERTY_HINT_ENUM, "Visible,Hidden,Hidden and Collapsed"), "set_dragger_visibility", "get_dragger_visibility");
	ADD_PROPERTY(PropertyInfo(Variant::BOOL, "vertical"), "set_vertical", "is_vertical");
	BIND_ENUM_CONSTANT(DRAGGER_VISIBLE);
	BIND_ENUM_CONSTANT(DRAGGER_HIDDEN);
	BIND_ENUM_CONSTANT(DRAGGER_HIDDEN_COLLAPSED);
}
SplitContainer::SplitContainer(bool p_vertical) {
	vertical = p_vertical;
	dragging_area_control = memnew(SplitContainerDragger);
	add_child(dragging_area_control, false, Node::INTERNAL_MODE_BACK);
}