### 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 function_color = dark_theme ? Color(0.34, 0.7, 1.0) : Color(0, 0.225, 0.9, 1);
	const Color member_variable_color = dark_theme ? Color(0.34, 0.7, 1.0).lerp(mono_color, 0.6) : Color(0, 0.4, 0.68, 1);
	const Color mark_color = Color(error_color.r, error_color.g, error_color.b, 0.3);
	const Color bookmark_color = Color(0.08, 0.49, 0.98);
	const Color breakpoint_color = dark_theme ? error_color : Color(1, 0.27, 0.2, 1);
	const Color executing_line_color = Color(0.98, 0.89, 0.27);
	const Color code_folding_color = alpha3;
	const Color search_result_color = alpha1;
	const Color search_result_border_color = dark_theme ? Color(0.41, 0.61, 0.91, 0.38) : Color(0, 0.4, 1, 0.38);
	EditorSettings *setting = EditorSettings::get_singleton();
	String text_editor_color_theme = setting->get("text_editor/theme/color_theme");
	if (text_editor_color_theme == "Default") {
		setting->set_initial_value("text_editor/theme/highlighting/symbol_color", symbol_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/keyword_color", keyword_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/control_flow_keyword_color", control_flow_keyword_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/base_type_color", base_type_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/engine_type_color", engine_type_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/user_type_color", user_type_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/comment_color", comment_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/string_color", string_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/background_color", te_background_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/completion_background_color", completion_background_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/completion_selected_color", completion_selected_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/completion_existing_color", completion_existing_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/completion_scroll_color", completion_scroll_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/completion_scroll_hovered_color", completion_scroll_hovered_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/completion_font_color", completion_font_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/text_color", text_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/line_number_color", line_number_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/safe_line_number_color", safe_line_number_color, true);