### 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("camera", "GraphEditMinimap", style_minimap_camera);
	theme->set_stylebox("node", "GraphEditMinimap", style_minimap_node);
	Color minimap_resizer_color;
	if (dark_theme) {
		minimap_resizer_color = Color(1, 1, 1, 0.65);
	} else {
		minimap_resizer_color = Color(0, 0, 0, 0.65);
	}
	theme->set_icon("resizer", "GraphEditMinimap", theme->get_icon(SNAME("GuiResizerTopLeft"), SNAME("EditorIcons")));
	theme->set_color("resizer_color", "GraphEditMinimap", minimap_resizer_color);
	
	const int gn_margin_side = 2;
	const int gn_margin_bottom = 2;
	
	const int sm_margin_side = 10;
	Color graphnode_bg = dark_color_3;
	if (!dark_theme) {
		graphnode_bg = prop_section_color;
	}
	Ref<StyleBoxFlat> graphsb = make_flat_stylebox(graphnode_bg.lerp(style_tree_bg->get_bg_color(), 0.3), gn_margin_side, 24, gn_margin_side, gn_margin_bottom, corner_width);
	graphsb->set_border_width_all(border_width);
	graphsb->set_border_color(graphnode_bg);
	Ref<StyleBoxFlat> graphsbselected = make_flat_stylebox(graphnode_bg * Color(1, 1, 1, 1), gn_margin_side, 24, gn_margin_side, gn_margin_bottom, corner_width);
	graphsbselected->set_border_width_all(2 * EDSCALE + border_width);
	graphsbselected->set_border_color(Color(accent_color.r, accent_color.g, accent_color.b, 0.6));
	Ref<StyleBoxFlat> graphsbcomment = make_flat_stylebox(graphnode_bg * Color(1, 1, 1, 0.3), gn_margin_side, 24, gn_margin_side, gn_margin_bottom, corner_width);
	graphsbcomment->set_border_width_all(border_width);
	graphsbcomment->set_border_color(graphnode_bg);
	Ref<StyleBoxFlat> graphsbcommentselected = make_flat_stylebox(graphnode_bg * Color(1, 1, 1, 0.4), gn_margin_side, 24, gn_margin_side, gn_margin_bottom, corner_width);
	graphsbcommentselected->set_border_width_all(border_width);