### 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)

}
			}
			p_theme->set_icon(editor_icons_names[index], SNAME("EditorIcons"), icon);
		}
	} else {
		const float scale = (float)p_thumb_size / 32.0 * EDSCALE;
		for (int i = 0; i < editor_md_thumbs_count; i++) {
			const int index = editor_md_thumbs_indices[i];
			Ref<ImageTexture> icon;
			if (accent_color_icons.has(editor_icons_names[index])) {
				icon = editor_generate_icon(index, scale, 1.0, accent_color_map);
			} else {
				float saturation = p_icon_saturation;
				if (saturation_exceptions.has(editor_icons_names[index])) {
					saturation = 1.0;
				}
				if (conversion_exceptions.has(editor_icons_names[index])) {
					icon = editor_generate_icon(index, scale, saturation);
				} else {
					icon = editor_generate_icon(index, scale, saturation, color_conversion_map);
				}
			}
			p_theme->set_icon(editor_icons_names[index], SNAME("EditorIcons"), icon);
		}
	}
#else
	WARN_PRINT("SVG support disabled, editor icons won't be rendered.");
#endif
}
Ref<Theme> create_editor_theme(const Ref<Theme> p_theme) {