### 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)

if (preset == "Custom") {
		accent_color = EDITOR_GET("interface/theme/accent_color");
		base_color = EDITOR_GET("interface/theme/base_color");
		contrast = EDITOR_GET("interface/theme/contrast");
	} else if (preset == "Breeze Dark") {
		preset_accent_color = Color(0.26, 0.76, 1.00);
		preset_base_color = Color(0.24, 0.26, 0.28);
		preset_contrast = default_contrast;
	} else if (preset == "Godot 2") {
		preset_accent_color = Color(0.53, 0.67, 0.89);
		preset_base_color = Color(0.24, 0.23, 0.27);
		preset_contrast = default_contrast;
	} else if (preset == "Gray") {
		preset_accent_color = Color(0.44, 0.73, 0.98);
		preset_base_color = Color(0.24, 0.24, 0.24);
		preset_contrast = default_contrast;
	} else if (preset == "Light") {
		preset_accent_color = Color(0.18, 0.50, 1.00);
		preset_base_color = Color(0.9, 0.9, 0.9);
		
		preset_contrast = -0.08;
	} else if (preset == "Solarized (Dark)") {
		preset_accent_color = Color(0.15, 0.55, 0.82);
		preset_base_color = Color(0.04, 0.23, 0.27);
		preset_contrast = default_contrast;
	} else if (preset == "Solarized (Light)") {
		preset_accent_color = Color(0.15, 0.55, 0.82);
		preset_base_color = Color(0.89, 0.86, 0.79);