### 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("#ff4545", "#ff2929"); 
	add_conversion_color_pair("#ffe345", "#ffe337"); 
	add_conversion_color_pair("#80ff45", "#74ff34"); 
	add_conversion_color_pair("#45ffa2", "#2cff98"); 
	add_conversion_color_pair("#45d7ff", "#22ccff"); 
	add_conversion_color_pair("#8045ff", "#702aff"); 
	add_conversion_color_pair("#ff4596", "#ff2781"); 
	
	add_conversion_color_pair("#e1da5b", "#d6cf4b"); 
	add_conversion_color_pair("#62aeff", "#1678e0"); 
	add_conversion_color_pair("#75d1e6", "#41acc5"); 
	add_conversion_color_pair("#84ffee", "#49ccba"); 
	add_conversion_color_pair("#f70000", "#c91616"); 
	add_conversion_color_pair("#eec315", "#d58c0b"); 
	add_conversion_color_pair("#dbee15", "#b7d10a"); 
	add_conversion_color_pair("#288027", "#218309"); 
	
	add_conversion_color_pair("#ffca5f", "#fea900"); 
	add_conversion_color_pair("#2998ff", "#68b6ff"); 
	add_conversion_color_pair("#a2d2ff", "#4998e3"); 
	
	
	add_conversion_color_pair("#ea7940", "#bd5e2c"); 
	add_conversion_color_pair("#ff2b88", "#bd165f"); 
	add_conversion_color_pair("#eac840", "#bd9d1f"); 
	add_conversion_color_pair("#3cf34e", "#16a827"); 
	add_conversion_color_pair("#2877f6", "#236be6"); 
	add_conversion_color_pair("#eae440", "#9f9722"); 
	add_conversion_color_pair("#a448f0", "#9853ce"); 
	add_conversion_color_pair("#5ad5c4", "#0a9c88");