Hello,
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For MasterDetailsView, it has MasterHeader property which can set the content for the master pane's header, and then you can override the MasterHeaderTemplate to reset the appearance of the header's content. For example:
.xaml:
<controls:MasterDetailsView
x:Name="MyMasterDetailsView"
BackButtonBehavior="Automatic"
CompactModeThresholdWidth="720"
ItemsSource="{x:Bind Emails}"
NoSelectionContent="Select an item to view"
MasterHeader="Inbox">
<controls:MasterDetailsView.MasterHeaderTemplate>
<DataTemplate>
<StackPanel>
<TextBlock FontSize="40" Text="{Binding}"></TextBlock>
</StackPanel>
</DataTemplate>
</controls:MasterDetailsView.MasterHeaderTemplate>
......
</controls:MasterDetailsView>