### 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_exception("CodeFoldedRightArrow");
	add_conversion_exception("CodeFoldDownArrow");
	add_conversion_exception("TextEditorPlay");
	add_conversion_exception("Breakpoint");
}
static Ref<StyleBoxTexture> make_stylebox(Ref<Texture2D> p_texture, float p_left, float p_top, float p_right, float p_bottom, float p_margin_left = -1, float p_margin_top = -1, float p_margin_right = -1, float p_margin_bottom = -1, bool p_draw_center = true) {
	Ref<StyleBoxTexture> style(memnew(StyleBoxTexture));
	style->set_texture(p_texture);
	style->set_texture_margin_individual(p_left * EDSCALE, p_top * EDSCALE, p_right * EDSCALE, p_bottom * EDSCALE);
	style->set_content_margin_individual((p_left + p_margin_left) * EDSCALE, (p_top + p_margin_top) * EDSCALE, (p_right + p_margin_right) * EDSCALE, (p_bottom + p_margin_bottom) * EDSCALE);
	style->set_draw_center(p_draw_center);
	return style;
}
static Ref<StyleBoxEmpty> make_empty_stylebox(float p_margin_left = -1, float p_margin_top = -1, float p_margin_right = -1, float p_margin_bottom = -1) {
	Ref<StyleBoxEmpty> style(memnew(StyleBoxEmpty));
	style->set_content_margin_individual(p_margin_left * EDSCALE, p_margin_top * EDSCALE, p_margin_right * EDSCALE, p_margin_bottom * EDSCALE);
	return style;
}
static Ref<StyleBoxFlat> make_flat_stylebox(Color p_color, float p_margin_left = -1, float p_margin_top = -1, float p_margin_right = -1, float p_margin_bottom = -1, int p_corner_width = 0) {
	Ref<StyleBoxFlat> style(memnew(StyleBoxFlat));
	style->set_bg_color(p_color);
	
	style->set_corner_detail(Math::ceil(0.8 * p_corner_width * EDSCALE));
	style->set_corner_radius_all(p_corner_width * EDSCALE);
	style->set_content_margin_individual(p_margin_left * EDSCALE, p_margin_top * EDSCALE, p_margin_right * EDSCALE, p_margin_bottom * EDSCALE);
	
	style->set_anti_aliased(false);
	return style;
}
static Ref<StyleBoxLine> make_line_stylebox(Color p_color, int p_thickness = 1, float p_grow_begin = 1, float p_grow_end = 1, bool p_vertical = false) {