### 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)

preset_contrast = -0.08;
	} else if (preset == "Black (OLED)") {
		preset_accent_color = Color(0.45, 0.75, 1.0);
		preset_base_color = Color(0, 0, 0);
		
		preset_contrast = 0.0;
		preset_draw_extra_borders = true;
	} else { 
		preset_accent_color = Color(0.44, 0.73, 0.98);
		preset_base_color = Color(0.21, 0.24, 0.29);
		preset_contrast = default_contrast;
	}
	if (preset != "Custom") {
		accent_color = preset_accent_color;
		base_color = preset_base_color;
		contrast = preset_contrast;
		draw_extra_borders = preset_draw_extra_borders;
		EditorSettings::get_singleton()->set_initial_value("interface/theme/accent_color", accent_color);
		EditorSettings::get_singleton()->set_initial_value("interface/theme/base_color", base_color);
		EditorSettings::get_singleton()->set_initial_value("interface/theme/contrast", contrast);
		EditorSettings::get_singleton()->set_initial_value("interface/theme/draw_extra_borders", draw_extra_borders);
	}
	EditorSettings::get_singleton()->set_manually("interface/theme/preset", preset);
	EditorSettings::get_singleton()->set_manually("interface/theme/accent_color", accent_color);
	EditorSettings::get_singleton()->set_manually("interface/theme/base_color", base_color);
	EditorSettings::get_singleton()->set_manually("interface/theme/contrast", contrast);
	EditorSettings::get_singleton()->set_manually("interface/theme/draw_extra_borders", draw_extra_borders);
	
	bool dark_theme = EditorSettings::get_singleton()->is_dark_theme();