### 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)

if (split_offset == p_offset) {
		return;
	}
	split_offset = p_offset;
	queue_sort();
}
int SplitContainer::get_split_offset() const {
	return split_offset;
}
void SplitContainer::clamp_split_offset() {
	if (!_getch(0) || !_getch(1)) {
		return;
	}
	_compute_middle_sep(true);
	queue_sort();
}
void SplitContainer::set_collapsed(bool p_collapsed) {