### 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)

theme->set_icon("folded", "CodeEdit", theme->get_icon(SNAME("CodeFoldedRightArrow"), SNAME("EditorIcons")));
	theme->set_icon("can_fold", "CodeEdit", theme->get_icon(SNAME("CodeFoldDownArrow"), SNAME("EditorIcons")));
	theme->set_icon("executing_line", "CodeEdit", theme->get_icon(SNAME("TextEditorPlay"), SNAME("EditorIcons")));
	theme->set_icon("breakpoint", "CodeEdit", theme->get_icon(SNAME("Breakpoint"), SNAME("EditorIcons")));
	theme->set_constant("line_spacing", "CodeEdit", EDITOR_GET("text_editor/appearance/whitespace/line_spacing"));
	theme->set_color("background_color", "CodeEdit", Color(0, 0, 0, 0));
	theme->set_color("completion_background_color", "CodeEdit", EDITOR_GET("text_editor/theme/highlighting/completion_background_color"));
	theme->set_color("completion_selected_color", "CodeEdit", EDITOR_GET("text_editor/theme/highlighting/completion_selected_color"));
	theme->set_color("completion_existing_color", "CodeEdit", EDITOR_GET("text_editor/theme/highlighting/completion_existing_color"));
	theme->set_color("completion_scroll_color", "CodeEdit", EDITOR_GET("text_editor/theme/highlighting/completion_scroll_color"));
	theme->set_color("completion_scroll_hovered_color", "CodeEdit", EDITOR_GET("text_editor/theme/highlighting/completion_scroll_hovered_color"));
	theme->set_color("completion_font_color", "CodeEdit", EDITOR_GET("text_editor/theme/highlighting/completion_font_color"));
	theme->set_color("font_color", "CodeEdit", EDITOR_GET("text_editor/theme/highlighting/text_color"));
	theme->set_color("line_number_color", "CodeEdit", EDITOR_GET("text_editor/theme/highlighting/line_number_color"));
	theme->set_color("caret_color", "CodeEdit", EDITOR_GET("text_editor/theme/highlighting/caret_color"));
	theme->set_color("font_selected_color", "CodeEdit", EDITOR_GET("text_editor/theme/highlighting/text_selected_color"));
	theme->set_color("selection_color", "CodeEdit", EDITOR_GET("text_editor/theme/highlighting/selection_color"));
	theme->set_color("brace_mismatch_color", "CodeEdit", EDITOR_GET("text_editor/theme/highlighting/brace_mismatch_color"));
	theme->set_color("current_line_color", "CodeEdit", EDITOR_GET("text_editor/theme/highlighting/current_line_color"));
	theme->set_color("line_length_guideline_color", "CodeEdit", EDITOR_GET("text_editor/theme/highlighting/line_length_guideline_color"));
	theme->set_color("word_highlighted_color", "CodeEdit", EDITOR_GET("text_editor/theme/highlighting/word_highlighted_color"));
	theme->set_color("bookmark_color", "CodeEdit", EDITOR_GET("text_editor/theme/highlighting/bookmark_color"));
	theme->set_color("breakpoint_color", "CodeEdit", EDITOR_GET("text_editor/theme/highlighting/breakpoint_color"));
	theme->set_color("executing_line_color", "CodeEdit", EDITOR_GET("text_editor/theme/highlighting/executing_line_color"));
	theme->set_color("code_folding_color", "CodeEdit", EDITOR_GET("text_editor/theme/highlighting/code_folding_color"));
	theme->set_color("search_result_color", "CodeEdit", EDITOR_GET("text_editor/theme/highlighting/search_result_color"));
	theme->set_color("search_result_border_color", "CodeEdit", EDITOR_GET("text_editor/theme/highlighting/search_result_border_color"));
	return theme;
}
Ref<Theme> create_custom_theme(const Ref<Theme> p_theme) {