### 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)

Ref<Theme> theme = create_editor_theme(p_theme);
	const String custom_theme_path = EDITOR_GET("interface/theme/custom_theme");
	if (!custom_theme_path.is_empty()) {
		Ref<Theme> custom_theme = ResourceLoader::load(custom_theme_path);
		if (custom_theme.is_valid()) {
			theme->merge_with(custom_theme);
		}
	}
	return theme;
}
/**
 * Returns the SVG code for the default project icon.
 */
String get_default_project_icon() {
	for (int i = 0; i < editor_icons_count; i++) {
		if (strcmp(editor_icons_names[i], "DefaultProjectIcon") == 0) {
			return String(editor_icons_sources[i]);
		}
	}
	return String();
}