### 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)

color_conversion_exceptions.insert(p_icon_name);
}
void EditorColorMap::create() {
	
	
	
	add_conversion_color_pair("#478cbf", "#478cbf"); 
	add_conversion_color_pair("#414042", "#414042"); 
	add_conversion_color_pair("#ffffff", "#414141"); 
	add_conversion_color_pair("#000000", "#bfbfbf"); 
	
	add_conversion_color_pair("#ff0000", "#ff0000"); 
	add_conversion_color_pair("#00ff00", "#00ff00"); 
	add_conversion_color_pair("#0000ff", "#0000ff"); 
	
	add_conversion_color_pair("#e0e0e0", "#5a5a5a"); 
	add_conversion_color_pair("#fefefe", "#fefefe"); 
	add_conversion_color_pair("#808080", "#808080"); 
	add_conversion_color_pair("#b3b3b3", "#363636"); 
	add_conversion_color_pair("#699ce8", "#699ce8"); 
	add_conversion_color_pair("#f9f9f9", "#606060"); 
	add_conversion_color_pair("#c38ef1", "#a85de9"); 
	add_conversion_color_pair("#fc7f7f", "#cd3838"); 
	add_conversion_color_pair("#8da5f3", "#3d64dd"); 
	add_conversion_color_pair("#4b70ea", "#1a3eac"); 
	add_conversion_color_pair("#8eef97", "#2fa139"); 
	add_conversion_color_pair("#5fb2ff", "#0079f0"); 
	add_conversion_color_pair("#003e7a", "#2b74bb"); 
	add_conversion_color_pair("#f7f5cf", "#615f3a");