### 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)

HashSet<StringName> conversion_exceptions = EditorColorMap::get_color_conversion_exceptions();
	
	HashSet<StringName> saturation_exceptions;
	saturation_exceptions.insert("DefaultProjectIcon");
	saturation_exceptions.insert("Godot");
	saturation_exceptions.insert("Logo");
	
	
	
	HashMap<Color, Color> accent_color_map;
	HashSet<StringName> accent_color_icons;
	const Color accent_color = p_theme->get_color(SNAME("accent_color"), SNAME("Editor"));
	accent_color_map[Color::html("699ce8")] = accent_color;
	if (accent_color.get_luminance() > 0.75) {
		accent_color_map[Color::html("ffffff")] = Color(0.2, 0.2, 0.2);
	}
	accent_color_icons.insert("GuiChecked");
	accent_color_icons.insert("GuiRadioChecked");
	accent_color_icons.insert("GuiIndeterminate");
	accent_color_icons.insert("GuiToggleOn");
	accent_color_icons.insert("GuiToggleOnMirrored");
	accent_color_icons.insert("PlayOverlay");
	
	if (!p_only_thumbs) {
		for (int i = 0; i < editor_icons_count; i++) {
			Ref<ImageTexture> icon;
			if (accent_color_icons.has(editor_icons_names[i])) {
				icon = editor_generate_icon(i, EDSCALE, 1.0, accent_color_map);
			} else {
				float saturation = p_icon_saturation;