### 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)

const Color icon_normal_color = Color(1, 1, 1);
	Color icon_hover_color = icon_normal_color * (dark_theme ? 1.15 : 1.45);
	icon_hover_color.a = 1.0;
	Color icon_focus_color = icon_hover_color;
	Color icon_disabled_color = Color(icon_normal_color, 0.4);
	
	
	Color icon_pressed_color = accent_color * (dark_theme ? 1.15 : 3.5);
	icon_pressed_color.a = 1.0;
	const Color separator_color = Color(mono_color.r, mono_color.g, mono_color.b, 0.1);
	const Color highlight_color = Color(accent_color.r, accent_color.g, accent_color.b, 0.275);
	const Color disabled_highlight_color = highlight_color.lerp(dark_theme ? Color(0, 0, 0) : Color(1, 1, 1), 0.5);
	
	theme->set_color("icon_saturation", "Editor", Color(icon_saturation, icon_saturation, icon_saturation));
	theme->set_color("accent_color", "Editor", accent_color);
	theme->set_color("highlight_color", "Editor", highlight_color);
	theme->set_color("disabled_highlight_color", "Editor", disabled_highlight_color);
	theme->set_color("base_color", "Editor", base_color);
	theme->set_color("dark_color_1", "Editor", dark_color_1);
	theme->set_color("dark_color_2", "Editor", dark_color_2);
	theme->set_color("dark_color_3", "Editor", dark_color_3);
	theme->set_color("contrast_color_1", "Editor", contrast_color_1);
	theme->set_color("contrast_color_2", "Editor", contrast_color_2);
	theme->set_color("box_selection_fill_color", "Editor", accent_color * Color(1, 1, 1, 0.3));
	theme->set_color("box_selection_stroke_color", "Editor", accent_color * Color(1, 1, 1, 0.8));
	theme->set_color("axis_x_color", "Editor", Color(0.96, 0.20, 0.32));
	theme->set_color("axis_y_color", "Editor", Color(0.53, 0.84, 0.01));
	theme->set_color("axis_z_color", "Editor", Color(0.16, 0.55, 0.96));
	theme->set_color("axis_w_color", "Editor", Color(0.55, 0.55, 0.55));
	const float prop_color_saturation = accent_color.get_s() * 0.75;