### 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)

add_conversion_color_pair("#eac968", "#d9b64f"); 
	add_conversion_color_pair("#cf68ea", "#c050dd"); 
	
	add_conversion_exception("EditorPivot");
	add_conversion_exception("EditorHandle");
	add_conversion_exception("Editor3DHandle");
	add_conversion_exception("EditorBoneHandle");
	add_conversion_exception("Godot");
	add_conversion_exception("Sky");
	add_conversion_exception("EditorControlAnchor");
	add_conversion_exception("DefaultProjectIcon");
	add_conversion_exception("GuiChecked");
	add_conversion_exception("GuiRadioChecked");
	add_conversion_exception("GuiIndeterminate");
	add_conversion_exception("GuiCloseCustomizable");
	add_conversion_exception("GuiGraphNodePort");
	add_conversion_exception("GuiResizer");
	add_conversion_exception("ZoomMore");
	add_conversion_exception("ZoomLess");
	add_conversion_exception("ZoomReset");
	add_conversion_exception("LockViewport");
	add_conversion_exception("GroupViewport");
	add_conversion_exception("StatusError");
	add_conversion_exception("StatusSuccess");
	add_conversion_exception("StatusWarning");
	add_conversion_exception("OverbrightIndicator");
	add_conversion_exception("GuiMiniCheckerboard");
	
	add_conversion_exception("GuiTab");
	add_conversion_exception("GuiSpace");