### 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)

const Color alpha2 = Color(mono_value, mono_value, mono_value, 0.14);
	const Color alpha3 = Color(mono_value, mono_value, mono_value, 0.27);
	const Color symbol_color = dark_theme ? Color(0.67, 0.79, 1) : Color(0, 0, 0.61);
	const Color keyword_color = dark_theme ? Color(1.0, 0.44, 0.52) : Color(0.9, 0.135, 0.51);
	const Color control_flow_keyword_color = dark_theme ? Color(1.0, 0.55, 0.8) : Color(0.743, 0.12, 0.8);
	const Color base_type_color = dark_theme ? Color(0.26, 1.0, 0.76) : Color(0, 0.6, 0.2);
	const Color engine_type_color = dark_theme ? Color(0.56, 1, 0.86) : Color(0.11, 0.55, 0.4);
	const Color user_type_color = dark_theme ? Color(0.78, 1, 0.93) : Color(0.18, 0.45, 0.4);
	const Color comment_color = dark_theme ? dim_color : Color(0.08, 0.08, 0.08, 0.5);
	const Color string_color = dark_theme ? Color(1, 0.93, 0.63) : Color(0.6, 0.42, 0);
	
	const Color te_background_color = dark_theme ? background_color : dark_color_3;
	const Color completion_background_color = dark_theme ? base_color : background_color;
	const Color completion_selected_color = alpha1;
	const Color completion_existing_color = alpha2;
	
	const Color completion_scroll_color = Color(mono_value, mono_value, mono_value, 0.29);
	const Color completion_scroll_hovered_color = Color(mono_value, mono_value, mono_value, 0.4);
	const Color completion_font_color = font_color;
	const Color text_color = font_color;
	const Color line_number_color = dim_color;
	const Color safe_line_number_color = dark_theme ? (dim_color * Color(1, 1.2, 1, 1.5)) : Color(0, 0.4, 0, 0.75);
	const Color caret_color = mono_color;
	const Color caret_background_color = mono_color.inverted();
	const Color text_selected_color = Color(0, 0, 0, 0);
	const Color brace_mismatch_color = dark_theme ? error_color : Color(1, 0.08, 0, 1);
	const Color current_line_color = alpha1;
	const Color line_length_guideline_color = dark_theme ? base_color : background_color;
	const Color word_highlighted_color = alpha1;
	const Color number_color = dark_theme ? Color(0.63, 1, 0.88) : Color(0, 0.55, 0.28, 1);