### 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
	if (dark_theme) {
		ImageLoaderSVG::set_forced_color_map(HashMap<Color, Color>());
	} else {
		ImageLoaderSVG::set_forced_color_map(EditorColorMap::get_color_conversion_map());
	}
#endif
	
	
	const Color dark_color_1 = base_color.lerp(Color(0, 0, 0, 1), contrast).clamp();
	const Color dark_color_2 = base_color.lerp(Color(0, 0, 0, 1), contrast * 1.5).clamp();
	const Color dark_color_3 = base_color.lerp(Color(0, 0, 0, 1), contrast * 2).clamp();
	
	const Color extra_border_color_1 = Color(0.5, 0.5, 0.5);
	const Color extra_border_color_2 = dark_theme ? Color(0.3, 0.3, 0.3) : Color(0.7, 0.7, 0.7);
	const Color background_color = dark_color_2;
	
	const Color mono_color = dark_theme ? Color(1, 1, 1) : Color(0, 0, 0);
	const Color contrast_color_1 = base_color.lerp(mono_color, MAX(contrast, default_contrast));
	const Color contrast_color_2 = base_color.lerp(mono_color, MAX(contrast * 1.5, default_contrast * 1.5));
	const Color font_color = mono_color.lerp(base_color, 0.25);
	const Color font_hover_color = mono_color.lerp(base_color, 0.125);
	const Color font_focus_color = mono_color.lerp(base_color, 0.125);
	const Color font_hover_pressed_color = font_hover_color.lerp(accent_color, 0.74);
	const Color font_disabled_color = Color(mono_color.r, mono_color.g, mono_color.b, 0.35);
	const Color font_readonly_color = Color(mono_color.r, mono_color.g, mono_color.b, 0.65);
	const Color font_placeholder_color = Color(mono_color.r, mono_color.g, mono_color.b, 0.6);
	const Color selection_color = accent_color * Color(1, 1, 1, 0.4);
	const Color disabled_color = mono_color.inverted().lerp(base_color, 0.7);
	const Color disabled_bg_color = mono_color.inverted().lerp(base_color, 0.9);