Core: Implement "follow system theme" / automatic theme switch AKA Automatically change the theme depending on the system theme

### Problem description

### Description
I am using "Auto Dark Mode" on windows and scheduled theme switch on Arch with GNOME. I've been looking around a bit in FreeCAD, but as it stands it seems like currently there is no "follow system theme" option, only light/dark theme.
Since FreeCAD allows the user to select custom themes, it would be nice to assign a theme to "light mode", one to "dark mode", so a "follow system" mode would be able to switch between the user theme of choice instead of the default themes.
My suggestion is that the wizard shows a "follow system" mode too

### Mockup

(btw the light and dark theme images have their icons misaligned, noticed that whilst making the mockup)

### Workbench affected?

None

### Steps to reproduce

obsolete for feature request

### Expected behavior

obsolete for feature request

### Actual behavior

obsolete for feature request

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

```shell
OS: Windows 10 build 19044
Architecture: x86_64
Version: 1.1.0.20260325 (Git shallow)
Build date: 2026/03/25 02:17:53
Build type: Release
Branch: (HEAD detached at 34a971666)
Hash: 34a9716668b1ddeb55b914f1c5be644826bdbbbf
Python 3.11.15, Qt 6.8.3, Coin 4.0.3, Vtk 9.3.1, boost 1_86, Eigen3 3.4.0, PySide 6.8.3
shiboken 6.8.3, xerces-c 3.3.0, IfcOpenShell 0.8.4, OCC 7.8.1
Locale: English/United States (en_US)
Navigation Style/Orbit Style/Rotation Mode: CAD/Rounded Arcball/Window center
Stylesheet/Theme/QtStyle: FreeCAD.qss/FreeCAD Light/
Logical DPI/Physical DPI/Pixel Ratio: 144/109.734/1
```

### Last known good version (optional)

```shell

```

Start by locating the theme-selection wizard and the existing light/dark and custom-theme handling in FreeCAD's desktop UI. Check how the application can observe system theme changes on Windows and GNOME, then define the light and dark user-theme assignments. Done means a follow-system option switches between those assigned themes when the system theme changes.

Written by the indexing model from the issue text.