### 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)

theme->set_color("highend_color", "Editor", Color(1.0, 0.0, 0.0));
	}
	const int thumb_size = EDITOR_GET("filesystem/file_dialog/thumbnail_size");
	theme->set_constant("scale", "Editor", EDSCALE);
	theme->set_constant("thumb_size", "Editor", thumb_size);
	theme->set_constant("dark_theme", "Editor", dark_theme);
	theme->set_constant("color_picker_button_height", "Editor", 28 * EDSCALE);
	
	
	
	bool keep_old_icons = false;
	bool regenerate_thumb_icons = true;
	if (p_theme != nullptr) {
		
		
		
		const float prev_scale = (float)p_theme->get_constant(SNAME("scale"), SNAME("Editor"));
		const bool prev_dark_theme = (bool)p_theme->get_constant(SNAME("dark_theme"), SNAME("Editor"));
		const Color prev_accent_color = p_theme->get_color(SNAME("accent_color"), SNAME("Editor"));
		const float prev_icon_saturation = p_theme->get_color(SNAME("icon_saturation"), SNAME("Editor")).r;
		keep_old_icons = (Math::is_equal_approx(prev_scale, EDSCALE) &&
				prev_dark_theme == dark_theme &&
				prev_accent_color == accent_color &&
				prev_icon_saturation == icon_saturation);
		const double prev_thumb_size = (double)p_theme->get_constant(SNAME("thumb_size"), SNAME("Editor"));
		regenerate_thumb_icons = !Math::is_equal_approx(prev_thumb_size, thumb_size);
	}
	if (keep_old_icons) {
		for (int i = 0; i < editor_icons_count; i++) {
			theme->set_icon(editor_icons_names[i], SNAME("EditorIcons"), p_theme->get_icon(editor_icons_names[i], SNAME("EditorIcons")));