### 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("folder_icon_color", "FileDialog", (dark_theme ? Color(1, 1, 1) : Color(4.25, 4.25, 4.25)).lerp(accent_color, 0.7));
	theme->set_color("files_disabled", "FileDialog", font_disabled_color);
	
	theme->set_constant("margin", "ColorPicker", popup_margin_size);
	theme->set_constant("sv_width", "ColorPicker", 256 * EDSCALE);
	theme->set_constant("sv_height", "ColorPicker", 256 * EDSCALE);
	theme->set_constant("h_width", "ColorPicker", 30 * EDSCALE);
	theme->set_constant("label_width", "ColorPicker", 10 * EDSCALE);
	theme->set_icon("screen_picker", "ColorPicker", theme->get_icon(SNAME("ColorPick"), SNAME("EditorIcons")));
	theme->set_icon("shape_circle", "ColorPicker", theme->get_icon(SNAME("PickerShapeCircle"), SNAME("EditorIcons")));
	theme->set_icon("shape_rect", "ColorPicker", theme->get_icon(SNAME("PickerShapeRectangle"), SNAME("EditorIcons")));
	theme->set_icon("shape_rect_wheel", "ColorPicker", theme->get_icon(SNAME("PickerShapeRectangleWheel"), SNAME("EditorIcons")));
	theme->set_icon("add_preset", "ColorPicker", theme->get_icon(SNAME("Add"), SNAME("EditorIcons")));
	theme->set_icon("sample_bg", "ColorPicker", theme->get_icon(SNAME("GuiMiniCheckerboard"), SNAME("EditorIcons")));
	theme->set_icon("overbright_indicator", "ColorPicker", theme->get_icon(SNAME("OverbrightIndicator"), SNAME("EditorIcons")));
	theme->set_icon("bar_arrow", "ColorPicker", theme->get_icon(SNAME("ColorPickerBarArrow"), SNAME("EditorIcons")));
	theme->set_icon("picker_cursor", "ColorPicker", theme->get_icon(SNAME("PickerCursor"), SNAME("EditorIcons")));
	
	theme->set_icon("bg", "ColorPickerButton", theme->get_icon(SNAME("GuiMiniCheckerboard"), SNAME("EditorIcons")));
	
	Ref<StyleBoxFlat> preset_sb = make_flat_stylebox(Color(1, 1, 1), 2, 2, 2, 2, 2);
	theme->set_stylebox("preset_fg", "ColorPresetButton", preset_sb);
	theme->set_icon("preset_bg", "ColorPresetButton", theme->get_icon(SNAME("GuiMiniCheckerboard"), SNAME("EditorIcons")));
	theme->set_icon("overbright_indicator", "ColorPresetButton", theme->get_icon(SNAME("OverbrightIndicator"), SNAME("EditorIcons")));
	
	Ref<StyleBoxFlat> style_info_3d_viewport = style_default->duplicate();
	style_info_3d_viewport->set_bg_color(style_info_3d_viewport->get_bg_color() * Color(1, 1, 1, 0.5));
	style_info_3d_viewport->set_border_width_all(0);
	theme->set_stylebox("Information3dViewport", "EditorStyles", style_info_3d_viewport);