Use external resource
<UserControl x:Class='SilverlightApplication3.MainPage' xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation' xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml' xmlns:d='http://schemas.microsoft.com/expression/blend/2008' xmlns:mc='http://schemas.openxmlformats.org/markup-compatibility/2006' mc:Ignorable='d' d:DesignWidth='640' d:DesignHeight='480'> <Grid x:Name="LayoutRoot" Background="White"> <Grid.Resources> <ResourceDictionary> <ResourceDictionary.MergedDictionaries> <ResourceDictionary Source="external.xaml" /> </ResourceDictionary.MergedDictionaries> <Style x:Name="Title" TargetType="TextBlock"> <Setter Property="FontFamily" Value="Calibri" /> <Setter Property="FontSize" Value="15" /> <Setter Property="FontWeight" Value="Bold"/> </Style> <Style x:Name="MainTitle" TargetType="TextBlock" BasedOn="{StaticResource Title}"> <Setter Property="Foreground" Value="Blue" /> </Style> <Style x:Name="SubTitle" TargetType="TextBlock" BasedOn="{StaticResource Title}"> <Setter Property="FontSize" Value="12" /> </Style> <Style x:Name="DynamicTitle" TargetType="TextBlock" BasedOn="{StaticResource Title}"> <Setter Property="FontSize" Value="20" /> <Setter Property="Foreground" Value="Green"/> </Style> </ResourceDictionary> </Grid.Resources> <StackPanel> <TextBlock Text="Title" Style="{StaticResource Title}" Margin="5"/> <TextBlock Text="Main Title" Style="{StaticResource MainTitle}" Margin="5"/> <TextBlock Text="Sub Title" Style="{StaticResource LargeTitle}" Margin="5"/> <Button x:Name="button1" Click="button1_Click" Content="Toggle Style" Width="150"/> <TextBlock x:Name="textBlock1" Text="Main Title" Style="{StaticResource MainTitle}" Margin="5"/> </StackPanel> </Grid> </UserControl> //File: Page.xaml.cs using System; using System.Collections.Generic; using System.Linq; using System.Net; using System.Windows; using System.Windows.Controls; using System.Windows.Documents; using System.Windows.Input; using System.Windows.Media; using System.Windows.Media.Animation; using System.Windows.Shapes; namespace SilverlightApplication3 { public partial class MainPage : UserControl { private bool isToggle; public MainPage() { InitializeComponent(); } private void button1_Click(object sender, RoutedEventArgs e) { if (isToggle == false) { textBlock1.Style = LayoutRoot.Resources["DynamicTitle"] as Style; } else { textBlock1.Style = LayoutRoot. Resources["MainTitle"] as Style; } isToggle = !isToggle; } } }