### 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)

#include "split_container.h"
#include "label.h"
#include "margin_container.h"
void SplitContainerDragger::gui_input(const Ref<InputEvent> &p_event) {
	ERR_FAIL_COND(p_event.is_null());
	SplitContainer *sc = Object::cast_to<SplitContainer>(get_parent());
	if (sc->collapsed || !sc->_getch(0) || !sc->_getch(1) || sc->dragger_visibility != SplitContainer::DRAGGER_VISIBLE) {
		return;
	}
	Ref<InputEventMouseButton> mb = p_event;
	if (mb.is_valid()) {
		if (mb->get_button_index() == MouseButton::LEFT) {
			if (mb->is_pressed()) {
				sc->_compute_middle_sep(true);
				dragging = true;
				drag_ofs = sc->split_offset;
				if (sc->vertical) {
					drag_from = get_transform().xform(mb->get_position()).y;
				} else {
					drag_from = get_transform().xform(mb->get_position()).x;
				}