Do not load Theme.qml as Theme singleton, instead use BasicThemeDefinition
The Theme singleton has been deprecated for a long time. To not completely break things that may still be using it, this will return an instance of BasicThemeDefinition as Theme singleton, which should at least give some colors.
Unfortunately, doing this means older Theme.qml files fail to load in
BasicTheme as QQmlComponent will refuse to load anything with
pragma Singleton in it. To workaround this, we strip the file of any such
lines before loading it.