### 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_stylebox("slider", "VSlider", make_flat_stylebox(dark_color_3, default_margin_size / 2, 0, default_margin_size / 2, 0, corner_width));
	theme->set_stylebox("grabber_area", "VSlider", make_flat_stylebox(contrast_color_1, default_margin_size / 2, 0, default_margin_size / 2, 0, corner_width));
	theme->set_stylebox("grabber_area_highlight", "VSlider", make_flat_stylebox(contrast_color_1, default_margin_size / 2, 0, default_margin_size / 2, 0));
	theme->set_constant("grabber_offset", "VSlider", 0);
	
	theme->set_color("default_color", "RichTextLabel", font_color);
	theme->set_color("font_shadow_color", "RichTextLabel", Color(0, 0, 0, 0));
	theme->set_constant("shadow_offset_x", "RichTextLabel", 1 * EDSCALE);
	theme->set_constant("shadow_offset_y", "RichTextLabel", 1 * EDSCALE);
	theme->set_constant("shadow_outline_size", "RichTextLabel", 1 * EDSCALE);
	theme->set_stylebox("focus", "RichTextLabel", make_empty_stylebox());
	theme->set_stylebox("normal", "RichTextLabel", style_tree_bg);
	
	Ref<StyleBoxFlat> style_editor_help = style_default->duplicate();
	style_editor_help->set_bg_color(dark_color_2);
	style_editor_help->set_border_color(dark_color_3);
	theme->set_stylebox("background", "EditorHelp", style_editor_help);
	theme->set_color("title_color", "EditorHelp", accent_color);
	theme->set_color("headline_color", "EditorHelp", mono_color);
	theme->set_color("text_color", "EditorHelp", font_color);
	theme->set_color("comment_color", "EditorHelp", font_color * Color(1, 1, 1, 0.6));
	theme->set_color("symbol_color", "EditorHelp", font_color * Color(1, 1, 1, 0.6));
	theme->set_color("value_color", "EditorHelp", font_color * Color(1, 1, 1, 0.6));
	theme->set_color("qualifier_color", "EditorHelp", font_color * Color(1, 1, 1, 0.8));
	theme->set_color("type_color", "EditorHelp", accent_color.lerp(font_color, 0.5));
	theme->set_color("selection_color", "EditorHelp", accent_color * Color(1, 1, 1, 0.4));
	theme->set_color("link_color", "EditorHelp", accent_color.lerp(mono_color, 0.8));
	theme->set_color("code_color", "EditorHelp", accent_color.lerp(mono_color, 0.6));
	theme->set_color("kbd_color", "EditorHelp", accent_color.lerp(property_color, 0.6));
	theme->set_color("code_bg_color", "EditorHelp", dark_color_3);