### 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)

const float prop_color_value = accent_color.get_v();
	theme->set_color("property_color_x", "Editor", Color().from_hsv(0.0 / 3.0 + 0.05, prop_color_saturation, prop_color_value));
	theme->set_color("property_color_y", "Editor", Color().from_hsv(1.0 / 3.0 + 0.05, prop_color_saturation, prop_color_value));
	theme->set_color("property_color_z", "Editor", Color().from_hsv(2.0 / 3.0 + 0.05, prop_color_saturation, prop_color_value));
	theme->set_color("property_color_w", "Editor", Color().from_hsv(1.5 / 3.0 + 0.05, prop_color_saturation, prop_color_value));
	theme->set_color("font_color", "Editor", font_color);
	theme->set_color("highlighted_font_color", "Editor", font_hover_color);
	theme->set_color("disabled_font_color", "Editor", font_disabled_color);
	theme->set_color("readonly_font_color", "Editor", font_readonly_color);
	theme->set_color("mono_color", "Editor", mono_color);
	Color success_color = Color(0.45, 0.95, 0.5);
	Color warning_color = Color(1, 0.87, 0.4);
	Color error_color = Color(1, 0.47, 0.42);
	Color property_color = font_color.lerp(Color(0.5, 0.5, 0.5), 0.5);
	Color readonly_color = property_color.lerp(dark_theme ? Color(0, 0, 0) : Color(1, 1, 1), 0.25);
	Color readonly_warning_color = error_color.lerp(dark_theme ? Color(0, 0, 0) : Color(1, 1, 1), 0.25);
	if (!dark_theme) {
		
		success_color = success_color.lerp(mono_color, 0.35);
		warning_color = warning_color.lerp(mono_color, 0.35);
		error_color = error_color.lerp(mono_color, 0.25);
	}
	theme->set_color("success_color", "Editor", success_color);
	theme->set_color("warning_color", "Editor", warning_color);
	theme->set_color("error_color", "Editor", error_color);
	theme->set_color("property_color", "Editor", property_color);
	theme->set_color("readonly_color", "Editor", readonly_color);
	if (!dark_theme) {
		theme->set_color("highend_color", "Editor", Color::hex(0xad1128ff));
	} else {