### 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)

#ifdef MODULE_SVG_ENABLED
	
	
	
	
	
	
	
	
	HashMap<Color, Color> color_conversion_map;
	
	
	if (!p_dark_theme) {
		for (KeyValue<Color, Color> &E : EditorColorMap::get_color_conversion_map()) {
			color_conversion_map[E.key] = E.value;
		}
	}
	
	const Color error_color = p_theme->get_color(SNAME("error_color"), SNAME("Editor"));
	const Color success_color = p_theme->get_color(SNAME("success_color"), SNAME("Editor"));
	const Color warning_color = p_theme->get_color(SNAME("warning_color"), SNAME("Editor"));
	color_conversion_map[Color::html("#ff5f5f")] = error_color;
	color_conversion_map[Color::html("#5fff97")] = success_color;
	color_conversion_map[Color::html("#ffdd65")] = warning_color;