### 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)

}
	} else {
		editor_register_and_generate_icons(theme, dark_theme, icon_saturation, thumb_size, false);
	}
	if (regenerate_thumb_icons) {
		editor_register_and_generate_icons(theme, dark_theme, icon_saturation, thumb_size, true);
	}
	
	editor_register_fonts(theme);
	
	const int border_width = CLAMP(border_size, 0, 2) * MAX(1, EDSCALE);
	const int corner_width = CLAMP(corner_radius, 0, 6);
	const int default_margin_size = 4;
	const int margin_size_extra = default_margin_size + CLAMP(border_size, 0, 2);
	
	
	Ref<StyleBoxFlat> style_default = make_flat_stylebox(base_color, default_margin_size, default_margin_size, default_margin_size, default_margin_size, corner_width);
	style_default->set_border_width_all(border_width);
	style_default->set_border_color(base_color);
	
	const float extra_spacing = EDITOR_GET("interface/theme/additional_spacing");
	const Vector2 widget_default_margin = Vector2(extra_spacing + 6, extra_spacing + default_margin_size + 1) * EDSCALE;
	Ref<StyleBoxFlat> style_widget = style_default->duplicate();
	style_widget->set_content_margin_individual(widget_default_margin.x, widget_default_margin.y, widget_default_margin.x, widget_default_margin.y);
	style_widget->set_bg_color(dark_color_1);
	if (draw_extra_borders) {
		style_widget->set_border_width_all(Math::round(EDSCALE));
		style_widget->set_border_color(extra_border_color_1);
	} else {
		style_widget->set_border_color(dark_color_2);