Shotcut is already set to Fusion Dark.
Windows was already set to Windows Dark.
Set it to Windows Light.
Set it back to Windows Dark.
Changed Shotcut to System theme, it restarted, and still on System theme.
“System” on Windows is the old system - Vista & 7 - as provided by the UI library we use - and does not read the palette in modern Windows. It is similar to Fusion Light but not the same. I should probably rename this menu item on Windows. I think there might be a new theme in the library to correspond to modern Windows, but that might require a major upgrade, which is in progress.