See in below there is explicit style name is "MyCustomStyle" for TextBlock,so you can get this style with c# and apply like this.
App.xaml
<Application.Resources>
<Style x:Key="MyCustomStyle" TargetType="TextBlock">
<Setter Property="FontSize" Value="8" />
</Style>
</Application.Resources>
C
Txtblk.Style = (Style)App.Current.Resources["MyCustomstyle"];