Fresh installs and new profiles now default to the Fusion (dark) theme
instead of the platform-native theme. The old "Default" theme is
renamed "System" to better describe its purpose — using the OS-native
Qt style (windowsvista, macOS, etc.).
Existing users who had "Default" selected are automatically migrated
to "System" so they keep their platform-native styling. Users with
an empty or invalid theme name now fall back to Fusion.
