Can't use resources/ styles set in App.xaml.cs in a WPF page (System.Windows.Controls.Page)

[pushmlk]

This works in a WPF window (System.Windows.Window):
<TextBlock Text="This is a Fluent styled TextBlock" Style="{StaticResource CaptionTextBlockStyle}" />

but, not in a WPF page (System.Windows.Controls.Page)

In both cases my App.xaml.cs has the following markup:
<Application.Resources> <ResourceDictionary> <ResourceDictionary.MergedDictionaries> <ResourceDictionary Source="pack://application:,,,/PresentationFramework.Fluent;component/Themes/Fluent.xaml" /> </ResourceDictionary.MergedDictionaries> </ResourceDictionary> </Application.Resources>