Can't override V/HScrollBar's Grabber styleboxes with StyleBoxFlat/StyleBoxTexture

### Tested versions

- Reproducible in: 4.6rc2, 4.5.1stable

### System information

Godot v4.6.rc2 - Windows 10 (build 19045) - Multi-window, 1 monitor - Direct3D 12 (Mobile) - dedicated NVIDIA GeForce RTX 4070 (NVIDIA; 32.0.15.7628) - 12th Gen Intel(R) Core(TM) i5-12490F (12 threads) - 31.81 GiB memory

### Issue description

Can't use StyleBoxFlat or StyleBoxTexture to override Grabber's styleboxes (include grabber, grabber_highlight, grabber_pressed).

Demo:

https://github.com/user-attachments/assets/b129f69e-ee72-4d6d-ba91-eaae06654ea2

### Steps to reproduce

N/A

### Minimal reproduction project (MRP)

N/A

The issue names V/HScrollBar and the grabber, grabber_highlight, and grabber_pressed styleboxes but provides no source file, test, reproduction steps, or MRP. Start by reproducing the behavior using the linked demo and tracing the scrollbar theme/stylebox handling. Done means StyleBoxFlat and StyleBoxTexture overrides work for the listed grabber states.

Written by the indexing model from the issue text.