### 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)

graphsbcommentselected->set_border_color(graphnode_bg);
	Ref<StyleBoxFlat> graphsbbreakpoint = graphsbselected->duplicate();
	graphsbbreakpoint->set_draw_center(false);
	graphsbbreakpoint->set_border_color(warning_color);
	graphsbbreakpoint->set_shadow_color(warning_color * Color(1.0, 1.0, 1.0, 0.1));
	Ref<StyleBoxFlat> graphsbposition = graphsbselected->duplicate();
	graphsbposition->set_draw_center(false);
	graphsbposition->set_border_color(error_color);
	graphsbposition->set_shadow_color(error_color * Color(1.0, 1.0, 1.0, 0.2));
	Ref<StyleBoxEmpty> graphsbslot = make_empty_stylebox(12, 0, 12, 0);
	Ref<StyleBoxFlat> smgraphsb = make_flat_stylebox(dark_color_3 * Color(1, 1, 1, 0.7), sm_margin_side, 24, sm_margin_side, gn_margin_bottom, corner_width);
	smgraphsb->set_border_width_all(border_width);
	smgraphsb->set_border_color(graphnode_bg);
	Ref<StyleBoxFlat> smgraphsbselected = make_flat_stylebox(graphnode_bg * Color(1, 1, 1, 0.9), sm_margin_side, 24, sm_margin_side, gn_margin_bottom, corner_width);
	smgraphsbselected->set_border_width_all(2 * EDSCALE + border_width);
	smgraphsbselected->set_border_color(Color(accent_color.r, accent_color.g, accent_color.b, 0.9));
	smgraphsbselected->set_shadow_size(8 * EDSCALE);
	smgraphsbselected->set_shadow_color(shadow_color);
	graphsb->set_border_width(SIDE_TOP, 24 * EDSCALE);
	graphsbselected->set_border_width(SIDE_TOP, 24 * EDSCALE);
	graphsbcomment->set_border_width(SIDE_TOP, 24 * EDSCALE);
	graphsbcommentselected->set_border_width(SIDE_TOP, 24 * EDSCALE);
	graphsb->set_corner_detail(corner_radius * EDSCALE);
	graphsbselected->set_corner_detail(corner_radius * EDSCALE);
	graphsbcomment->set_corner_detail(corner_radius * EDSCALE);
	graphsbcommentselected->set_corner_detail(corner_radius * EDSCALE);
	theme->set_stylebox("frame", "GraphNode", graphsb);
	theme->set_stylebox("selected_frame", "GraphNode", graphsbselected);
	theme->set_stylebox("comment", "GraphNode", graphsbcomment);
	theme->set_stylebox("comment_focus", "GraphNode", graphsbcommentselected);