### 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)

Ref<Theme> theme = Ref<Theme>(memnew(Theme));
	
	theme->set_default_base_scale(EDSCALE);
	
	Color accent_color = EDITOR_GET("interface/theme/accent_color");
	Color base_color = EDITOR_GET("interface/theme/base_color");
	float contrast = EDITOR_GET("interface/theme/contrast");
	bool enable_touchscreen_touch_area = EDITOR_GET("interface/theme/enable_touchscreen_touch_area");
	bool draw_extra_borders = EDITOR_GET("interface/theme/draw_extra_borders");
	float icon_saturation = EDITOR_GET("interface/theme/icon_saturation");
	float relationship_line_opacity = EDITOR_GET("interface/theme/relationship_line_opacity");
	String preset = EDITOR_GET("interface/theme/preset");
	int border_size = EDITOR_GET("interface/theme/border_size");
	int corner_radius = EDITOR_GET("interface/theme/corner_radius");
	Color preset_accent_color;
	Color preset_base_color;
	float preset_contrast = 0;
	bool preset_draw_extra_borders = false;
	const float default_contrast = 0.3;