When applying Fluent design in WPF on .net9, the style of controls set in App.xaml does not adhere to the Fluent design.

### Description

When applying Fluent design in .NET 9 and specifying the common styles of the controls in 'Window.Resource', the styles are reflected correctly. However, if specified in the ResourceDictionary of App.xaml, it works fine in the Visual Studio design time, but compiles into an old style that is not Fluent.

### Reproduction Steps

This will work well.
```

<Setter Property="FontSize" Value="30"/>

```

This won't work well.
```

```

app.xaml
```

<Setter Property="FontSize" Value="30"/>

```

### Expected behavior

### Actual behavior

### Regression?

_No response_

### Known Workarounds

_No response_

### Impact

_No response_

### Configuration

_No response_

### Other information

_No response_