### 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)

return vertical;
}
Vector<int> SplitContainer::get_allowed_size_flags_horizontal() const {
	Vector<int> flags;
	flags.append(SIZE_FILL);
	if (!vertical) {
		flags.append(SIZE_EXPAND);
	}
	flags.append(SIZE_SHRINK_BEGIN);
	flags.append(SIZE_SHRINK_CENTER);
	flags.append(SIZE_SHRINK_END);
	return flags;
}
Vector<int> SplitContainer::get_allowed_size_flags_vertical() const {
	Vector<int> flags;
	flags.append(SIZE_FILL);
	if (vertical) {
		flags.append(SIZE_EXPAND);
	}
	flags.append(SIZE_SHRINK_BEGIN);
	flags.append(SIZE_SHRINK_CENTER);
	flags.append(SIZE_SHRINK_END);
	return flags;
}
void SplitContainer::_bind_methods() {