### 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_icon("close_pressed", "Window", theme->get_icon(SNAME("GuiClose"), SNAME("EditorIcons")));
	theme->set_constant("close_h_offset", "Window", 22 * EDSCALE);
	theme->set_constant("close_v_offset", "Window", 20 * EDSCALE);
	theme->set_constant("title_height", "Window", 24 * EDSCALE);
	theme->set_constant("resize_margin", "Window", 4 * EDSCALE);
	theme->set_font("title_font", "Window", theme->get_font(SNAME("title"), SNAME("EditorFonts")));
	theme->set_font_size("title_font_size", "Window", theme->get_font_size(SNAME("title_size"), SNAME("EditorFonts")));
	
	Ref<StyleBoxFlat> style_complex_window = style_window->duplicate();
	style_complex_window->set_bg_color(dark_color_2);
	style_complex_window->set_border_color(dark_color_2);
	theme->set_stylebox("panel", "EditorSettingsDialog", style_complex_window);
	theme->set_stylebox("panel", "ProjectSettingsEditor", style_complex_window);
	theme->set_stylebox("panel", "EditorAbout", style_complex_window);
	
	theme->set_stylebox("panel", "AcceptDialog", style_window_title);
	theme->set_constant("buttons_separation", "AcceptDialog", 8 * EDSCALE);
	
	Ref<Texture2D> empty_icon = memnew(ImageTexture);
	if (enable_touchscreen_touch_area) {
		theme->set_stylebox("scroll", "HScrollBar", make_line_stylebox(separator_color, 50));
	} else {
		theme->set_stylebox("scroll", "HScrollBar", make_stylebox(theme->get_icon(SNAME("GuiScrollBg"), SNAME("EditorIcons")), 5, 5, 5, 5, 1, 1, 1, 1));
	}
	theme->set_stylebox("scroll_focus", "HScrollBar", make_stylebox(theme->get_icon(SNAME("GuiScrollBg"), SNAME("EditorIcons")), 5, 5, 5, 5, 1, 1, 1, 1));
	theme->set_stylebox("grabber", "HScrollBar", make_stylebox(theme->get_icon(SNAME("GuiScrollGrabber"), SNAME("EditorIcons")), 6, 6, 6, 6, 1, 1, 1, 1));
	theme->set_stylebox("grabber_highlight", "HScrollBar", make_stylebox(theme->get_icon(SNAME("GuiScrollGrabberHl"), SNAME("EditorIcons")), 5, 5, 5, 5, 1, 1, 1, 1));
	theme->set_stylebox("grabber_pressed", "HScrollBar", make_stylebox(theme->get_icon(SNAME("GuiScrollGrabberPressed"), SNAME("EditorIcons")), 6, 6, 6, 6, 1, 1, 1, 1));
	theme->set_icon("increment", "HScrollBar", empty_icon);
	theme->set_icon("increment_highlight", "HScrollBar", empty_icon);