Bug: Theme preference resets on app restart

[ogi-m]

Summary:
Selected theme preference doesn't persist after app restart, instead it reverts to system default.

Steps to reproduce the behavior:

1. Launch Focalboard on Windows (the store app)
2. Click on Settings > Set theme
3. Select any theme other than System theme
4. Quit the app and start it again
5. Observe the theme

Expected behavior:
The theme selected in step 3 is displayed

Observed behavior:
App seems to have defaulted back to System theme

Platform:

• Windows 10
• Focalboard desktop app

[chenilim]

Dup of #314. This should already be fixed as part of a checkin for #314. Should be able to verify this in the latest build from source, or in the next release.