### 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)

} else {
		theme->set_color("grid_major", "GraphEdit", Color(0.0, 0.0, 0.0, 0.15));
		theme->set_color("grid_minor", "GraphEdit", Color(0.0, 0.0, 0.0, 0.07));
	}
	theme->set_color("selection_fill", "GraphEdit", theme->get_color(SNAME("box_selection_fill_color"), SNAME("Editor")));
	theme->set_color("selection_stroke", "GraphEdit", theme->get_color(SNAME("box_selection_stroke_color"), SNAME("Editor")));
	theme->set_color("activity", "GraphEdit", accent_color);
	theme->set_icon("minus", "GraphEdit", theme->get_icon(SNAME("ZoomLess"), SNAME("EditorIcons")));
	theme->set_icon("more", "GraphEdit", theme->get_icon(SNAME("ZoomMore"), SNAME("EditorIcons")));
	theme->set_icon("reset", "GraphEdit", theme->get_icon(SNAME("ZoomReset"), SNAME("EditorIcons")));
	theme->set_icon("snap", "GraphEdit", theme->get_icon(SNAME("SnapGrid"), SNAME("EditorIcons")));
	theme->set_icon("minimap", "GraphEdit", theme->get_icon(SNAME("GridMinimap"), SNAME("EditorIcons")));
	theme->set_icon("layout", "GraphEdit", theme->get_icon(SNAME("GridLayout"), SNAME("EditorIcons")));
	
	Ref<StyleBoxFlat> style_minimap_bg = make_flat_stylebox(dark_color_1, 0, 0, 0, 0);
	style_minimap_bg->set_border_color(dark_color_3);
	style_minimap_bg->set_border_width_all(1);
	theme->set_stylebox("bg", "GraphEditMinimap", style_minimap_bg);
	Ref<StyleBoxFlat> style_minimap_camera;
	Ref<StyleBoxFlat> style_minimap_node;
	if (dark_theme) {
		style_minimap_camera = make_flat_stylebox(Color(0.65, 0.65, 0.65, 0.2), 0, 0, 0, 0);
		style_minimap_camera->set_border_color(Color(0.65, 0.65, 0.65, 0.45));
		style_minimap_node = make_flat_stylebox(Color(1, 1, 1), 0, 0, 0, 0);
	} else {
		style_minimap_camera = make_flat_stylebox(Color(0.38, 0.38, 0.38, 0.2), 0, 0, 0, 0);
		style_minimap_camera->set_border_color(Color(0.38, 0.38, 0.38, 0.45));
		style_minimap_node = make_flat_stylebox(Color(0, 0, 0), 0, 0, 0, 0);
	}
	style_minimap_camera->set_border_width_all(1);