Gui: Theme choice not respected in safe mode

### Problem description

When starting safe mode, user is prompted for light or dark theme. Neither choice is respected, and instead FreeCAD uses Classic theme.

### Workbench affected?

Core (App, Gui,...)

### Steps to reproduce

Start FreeCAD in Safe Mode
Select any theme.
See that the theme choice was not respected.

### Expected behavior

Theme choice is applied.

### Actual behavior

Theme choice is not applied

### Development version About Info (in Safe Mode)

```shell
OS: Fedora Linux 43 (Sway) (sway/sway/wayland)
Architecture: x86_64
Version: 26.3.0dev.47825 (Git)
Build date: 2026/07/24 17:24:42
Build type: Debug
Branch: main
Hash: 0feb404d875e701c5d66a103864f5ed21b77212f
Python 3.11.14, Qt 6.8.3, Coin 4.0.10 (bundled), Pivy 0.6.11 (bundled), Vtk 9.3.1, boost 1_86, Eigen3 3.4.0, PySide 6.8.3
shiboken 6.8.3, xerces-c 3.3.0, Clipper2 , IfcOpenShell 0.8.2, OCC 7.8.1
Locale: English/Australia (en_AU)
Stylesheet/Theme/QtStyle: unset/unset/
QPA/File dialog/Color dialog: wayland/via Qt/via Qt
Navigation Style/Orbit Style/Rotation Mode: CAD/Rounded Arcball/Window center
Logical DPI/Physical DPI/Pixel Ratio: 96/159.34/1
OpenGL version/vendor/renderer: 4.6 (Compatibility Profile) Mesa 25.3.6/Intel/Mesa Intel(R) Iris(R) Xe Graphics (TGL GT2)
```

### Last known good version (optional)

```shell

```

Start by reproducing the issue in FreeCAD Safe Mode and inspect the GUI startup path where the light or dark theme choice is handled. Done means either selected theme is applied in Safe Mode instead of Classic, with coverage or verification for both choices.

Written by the indexing model from the issue text.