Checklist

• I have searched the issue tracker for open issues that relate to the same problem, before opening a new one.
• This issue only relates to a single bug. I will open new issues for any other problems.

Describe the bug

After using the Settings widget to change the global fyne appearance settings, the Theme (variant) selection is not obeyed on app start due to a bug in the JSON loading.

How to reproduce

1. Run the fyne demo on a system where it defaults to the Light theme
2. Use File->Settings to change the default theme to Dark
3. Quit the demo
4. Start the demo again
5. Observe it starts in light mode again
6. Use File->Settings to see the unapplied theme data (see screenshot)

Screenshots

Example code

N/A

Fyne version

2.6.0-beta1 or develop

Go compiler version

1.24

Operating system and version

Ubuntu 24.04

Additional Information

Checking released tags, this was introduced somewhere between v2.5.2 and v2.5.3.

A full git bisect indicates this bug was introduced in d4bbfc5 which github doesn't associate with any PR.

cc @Jacalz and @andydotxyz from that commit