### 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("#d6d6d6", "#474747"); 
	add_conversion_color_pair("#474747", "#d6d6d6"); 
	add_conversion_color_pair("#919191", "#6e6e6e"); 
	
	add_conversion_color_pair("#fce00e", "#aa8d24"); 
	add_conversion_color_pair("#0e71fc", "#0350bd"); 
	add_conversion_color_pair("#c6ced4", "#828f9b"); 
	
	add_conversion_color_pair("#41ecad", "#25e3a0"); 
	add_conversion_color_pair("#6f91f0", "#6d8eeb"); 
	add_conversion_color_pair("#5abbef", "#4fb2e9"); 
	add_conversion_color_pair("#35d4f4", "#27ccf0"); 
	add_conversion_color_pair("#4593ec", "#4690e7"); 
	add_conversion_color_pair("#ac73f1", "#ad76ee"); 
	add_conversion_color_pair("#f1738f", "#ee758e"); 
	add_conversion_color_pair("#de66f0", "#dc6aed"); 
	add_conversion_color_pair("#b9ec41", "#96ce1a"); 
	add_conversion_color_pair("#f74949", "#f77070"); 
	add_conversion_color_pair("#ec418e", "#ec69a3"); 
	add_conversion_color_pair("#ee5677", "#ee7991"); 
	add_conversion_color_pair("#e1ec41", "#b2bb19"); 
	add_conversion_color_pair("#f68f45", "#f49047"); 
	add_conversion_color_pair("#417aec", "#6993ec"); 
	add_conversion_color_pair("#41ec80", "#2ce573"); 
	add_conversion_color_pair("#55f3e3", "#12d5c3"); 
	add_conversion_color_pair("#54ed9e", "#57e99f"); 
	
	add_conversion_color_pair("#77ce57", "#67c046"); 
	add_conversion_color_pair("#ea686c", "#d95256");