### 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)

#include "editor_themes.h"
#include "core/error/error_macros.h"
#include "core/io/resource_loader.h"
#include "editor/editor_fonts.h"
#include "editor/editor_icons.gen.h"
#include "editor/editor_scale.h"
#include "editor/editor_settings.h"
#include "modules/modules_enabled.gen.h" 
#ifdef MODULE_SVG_ENABLED
#include "modules/svg/image_loader_svg.h"
#endif
HashMap<Color, Color> EditorColorMap::color_conversion_map;
HashSet<StringName> EditorColorMap::color_conversion_exceptions;
void EditorColorMap::add_conversion_color_pair(const String p_from_color, const String p_to_color) {
	color_conversion_map[Color::html(p_from_color)] = Color::html(p_to_color);
}
void EditorColorMap::add_conversion_exception(const StringName p_icon_name) {