PanelContainers don't render a default stylebox in the editor interface

### Tested versions

- Reproducible in 4.4.beta [de940706a]

### System information

Godot v4.4.beta (de940706a) - Windows 10 (build 19045) - Multi-window, 3 monitors - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 2080 SUPER (NVIDIA; 32.0.15.6094) - AMD Ryzen 9 5950X 16-Core Processor (32 threads)

### Issue description

PanelContainers without a overridden StyleBox don't render their StyleBox when in an editor panel.

I've only tested this in the main screen, but it likely will occur in other docks as well.

2D View

Game View

EditorInterface View

### Steps to reproduce

1. Create an EditorPlugin
2. Make said plugin use the main screen (_has_main_screen returns true, make it visible, etc.)
3. Add a PanelContainer to the scene that gets parented to the editor
4. Enable the plugin and view it in its dock

### Minimal reproduction project (MRP)

[mrp.zip](https://github.com/user-attachments/files/18595749/mrp.zip)

Start with the minimal reproduction project and reproduce the issue through an EditorPlugin whose _has_main_screen returns true, then inspect the editor path where its PanelContainer is parented and displayed. Done means the default StyleBox renders for the PanelContainer in the editor interface, including the main screen and any affected docks, without requiring an overridden StyleBox.

Written by the indexing model from the issue text.