Override ThemeResource for specific controls

I want to create a transparent TextBox, with no background color when focused (default is white, or SystemControlBackgroundChromeWhiteBrush). I understand that I can override the used color in the App.xaml like so:

<Application.Resources>
    <SolidColorBrush x:Key="SystemControlBackgroundChromeWhiteBrush">Transparent</SolidColorBrush>
</Application.Resources>

(or by using multiple ThemeDictionaries)

However I don't want to modify ALL TextBlocks but only specific ones. My first guess was to include the changed brush in the TextBlock's Resources, but this doesn't work.

Solution

You can modify default styles of control. HERE is the article how to find it.

Get style of TextBox, insert it as a resource in XAML and modify, eg.:

   <!-- Default style for Windows.UI.Xaml.Controls.TextBox -->
 <Style TargetType="TextBox">
<Setter Property="MinWidth" Value="{ThemeResource TextControlThemeMinWidth}" />
<Setter Property="MinHeight" Value="{ThemeResource TextControlThemeMinHeight}" />
<Setter Property="Foreground" Value="{ThemeResource TextBoxForegroundThemeBrush}" />
<Setter Property="SelectionHighlightColor" Value="{ThemeResource TextSelectionHighlightColorThemeBrush}" />
<Setter Property="Background" Value="{ThemeResource TextBoxBackgroundThemeBrush}" />
<Setter Property="BorderBrush" Value="{ThemeResource TextBoxBorderThemeBrush}" />
<Setter Property="BorderThickness" Value="{ThemeResource TextControlBorderThemeThickness}" />
<Setter Property="FontFamily" Value="{ThemeResource PhoneFontFamilyNormal}" />
<Setter Property="FontSize" Value="{ThemeResource ContentControlFontSize}" />
<Setter Property="TextWrapping" Value="NoWrap" />
<Setter Property="ScrollViewer.HorizontalScrollMode" Value="Auto" />
<Setter Property="ScrollViewer.VerticalScrollMode" Value="Auto" />
<Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Hidden" />
<Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Hidden" />
<Setter Property="ScrollViewer.IsDeferredScrollingEnabled" Value="False" />
<Setter Property="Padding" Value="{ThemeResource TextControlThemePadding}" />
<Setter Property="Margin" Value="{ThemeResource TextControlMarginThemeThickness}" />
<Setter Property="VerticalAlignment" Value="Top" />
<Setter Property="Template">
  <Setter.Value>
    <ControlTemplate TargetType="TextBox">
      <Grid Background="Transparent">
        <VisualStateManager.VisualStateGroups>
          <VisualStateGroup x:Name="CommonStates">
            <VisualState x:Name="Disabled">
              <Storyboard>
                <ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="Background">
                  <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxDisabledBackgroundThemeBrush}" />
                </ObjectAnimationUsingKeyFrames>
                <ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="BorderBrush">
                  <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxDisabledBorderThemeBrush}" />
                </ObjectAnimationUsingKeyFrames>
                <ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentElement" Storyboard.TargetProperty="Foreground">
                  <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxDisabledForegroundThemeBrush}" />
                </ObjectAnimationUsingKeyFrames>
                <ObjectAnimationUsingKeyFrames Storyboard.TargetName="PlaceholderTextContentPresenter" Storyboard.TargetProperty="Foreground">
                  <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxDisabledForegroundThemeBrush}" />
                </ObjectAnimationUsingKeyFrames>
                <ObjectAnimationUsingKeyFrames Storyboard.TargetName="HeaderContentPresenter" Storyboard.TargetProperty="Foreground">
                  <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxDisabledHeaderForegroundThemeBrush}" />
                </ObjectAnimationUsingKeyFrames>
              </Storyboard>
            </VisualState>
            <VisualState x:Name="Normal">
              <Storyboard>
                <DoubleAnimation Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="Opacity" Duration="0"
                                 To="{ThemeResource TextControlBorderThemeOpacity}" />
              </Storyboard>
            </VisualState>
            <VisualState x:Name="Focused">
              <Storyboard>
                <ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="BorderBrush">
                  <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextSelectionHighlightColorThemeBrush}" />
                </ObjectAnimationUsingKeyFrames>
                <DoubleAnimation Storyboard.TargetName="PlaceholderTextContentPresenter" Storyboard.TargetProperty="Opacity" Duration="0" To="0" />
                <ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="Background">
                  <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxFocusedBackgroundThemeBrush}" />
                </ObjectAnimationUsingKeyFrames>
              </Storyboard>
            </VisualState>
          </VisualStateGroup>
        </VisualStateManager.VisualStateGroups>
        <Grid.RowDefinitions>
          <RowDefinition Height="Auto"/>
          <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
        <Border x:Name="BorderElement" Grid.Row="1" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" />
        <ContentPresenter x:Name="HeaderContentPresenter" Grid.Row="0" Style="{StaticResource HeaderContentPresenterStyle}" Margin="{ThemeResource TextControlHeaderMarginThemeThickness}"
                          Content="{TemplateBinding Header}" ContentTemplate="{TemplateBinding HeaderTemplate}" />
        <ScrollViewer x:Name="ContentElement" Grid.Row="1" MinHeight="{ThemeResource TextControlThemeMinHeight}"
                      HorizontalScrollMode="{TemplateBinding ScrollViewer.HorizontalScrollMode}"
                      HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}"
                      VerticalScrollMode="{TemplateBinding ScrollViewer.VerticalScrollMode}"
                      VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}"
                      IsHorizontalRailEnabled="{TemplateBinding ScrollViewer.IsHorizontalRailEnabled}"
                      IsVerticalRailEnabled="{TemplateBinding ScrollViewer.IsVerticalRailEnabled}"
                      IsDeferredScrollingEnabled="{TemplateBinding ScrollViewer.IsDeferredScrollingEnabled}" Margin="{TemplateBinding BorderThickness}"
                      Padding="{TemplateBinding Padding}" IsTabStop="False" ZoomMode="Disabled"
                      AutomationProperties.AccessibilityView="Raw"/>
        <ContentControl x:Name="PlaceholderTextContentPresenter" Grid.Row="1" Foreground="{ThemeResource TextBoxPlaceholderTextThemeBrush}"
                        FontSize="{ThemeResource ContentControlFontSize}" Margin="{TemplateBinding BorderThickness}" Padding="{TemplateBinding Padding}"
                        IsTabStop="False" Content="{TemplateBinding PlaceholderText}" />
      </Grid>
    </ControlTemplate>
  </Setter.Value>
</Setter>

Next, add name to modified style:

<Style  x:Name="CustomTextBlock" TargetType="TextBox">

And apply it to your control, eg:

<TextBlock Style="{StaticResource CustomTextBlock}"/>

Here is the sample style for Transparent TextBox (WinRT/Universal Apps):

<Style x:Key="TransparentTextBox"
           TargetType="TextBox">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="TextBox">
                    <Grid Background="Transparent">
                        <Grid.RowDefinitions>
                            <RowDefinition Height="Auto" />
                            <RowDefinition Height="*" />
                        </Grid.RowDefinitions>
                        <VisualStateManager.VisualStateGroups>
                            <VisualStateGroup x:Name="CommonStates">
                                <VisualState x:Name="Disabled">
                                    <Storyboard>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush"
                                                                       Storyboard.TargetName="BorderElement">
                                            <DiscreteObjectKeyFrame KeyTime="0"
                                                                    Value="{ThemeResource TextBoxDisabledBorderThemeBrush}" />
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground"
                                                                       Storyboard.TargetName="ContentElement">
                                            <DiscreteObjectKeyFrame KeyTime="0"
                                                                    Value="{ThemeResource TextBoxDisabledForegroundThemeBrush}" />
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground"
                                                                       Storyboard.TargetName="PlaceholderTextContentPresenter">
                                            <DiscreteObjectKeyFrame KeyTime="0"
                                                                    Value="{ThemeResource TextBoxDisabledForegroundThemeBrush}" />
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground"
                                                                       Storyboard.TargetName="HeaderContentPresenter">
                                            <DiscreteObjectKeyFrame KeyTime="0"
                                                                    Value="{ThemeResource TextBoxDisabledHeaderForegroundThemeBrush}" />
                                        </ObjectAnimationUsingKeyFrames>
                                    </Storyboard>
                                </VisualState>
                                <VisualState x:Name="Normal">
                                    <Storyboard>
                                        <DoubleAnimation Duration="0"
                                                         To="{ThemeResource TextControlBorderThemeOpacity}"
                                                         Storyboard.TargetProperty="Opacity"
                                                         Storyboard.TargetName="BorderElement" />
                                    </Storyboard>
                                </VisualState>
                                <VisualState x:Name="Focused">
                                    <Storyboard>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush"
                                                                       Storyboard.TargetName="BorderElement">
                                            <DiscreteObjectKeyFrame KeyTime="0"
                                                                    Value="{ThemeResource TextSelectionHighlightColorThemeBrush}" />
                                        </ObjectAnimationUsingKeyFrames>
                                        <DoubleAnimation Duration="0"
                                                         To="0"
                                                         Storyboard.TargetProperty="Opacity"
                                                         Storyboard.TargetName="PlaceholderTextContentPresenter" />
                                    </Storyboard>
                                </VisualState>
                            </VisualStateGroup>
                        </VisualStateManager.VisualStateGroups>
                        <Border x:Name="BorderElement"
                                BorderBrush="{TemplateBinding BorderBrush}"
                                BorderThickness="{TemplateBinding BorderThickness}"
                                Grid.Row="1" />
                        <ContentPresenter x:Name="HeaderContentPresenter"
                                          ContentTemplate="{TemplateBinding HeaderTemplate}"
                                          Content="{TemplateBinding Header}"
                                          Margin="{ThemeResource TextControlHeaderMarginThemeThickness}"
                                          Grid.Row="0"
                                          Style="{StaticResource HeaderContentPresenterStyle}" />
                        <ScrollViewer x:Name="ContentElement"
                                      AutomationProperties.AccessibilityView="Raw"
                                      HorizontalScrollMode="{TemplateBinding ScrollViewer.HorizontalScrollMode}"
                                      HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}"
                                      IsTabStop="False"
                                      IsHorizontalRailEnabled="{TemplateBinding ScrollViewer.IsHorizontalRailEnabled}"
                                      IsVerticalRailEnabled="{TemplateBinding ScrollViewer.IsVerticalRailEnabled}"
                                      IsDeferredScrollingEnabled="{TemplateBinding ScrollViewer.IsDeferredScrollingEnabled}"
                                      Margin="{TemplateBinding BorderThickness}"
                                      MinHeight="{ThemeResource TextControlThemeMinHeight}"
                                      Padding="{TemplateBinding Padding}"
                                      Grid.Row="1"
                                      VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}"
                                      VerticalScrollMode="{TemplateBinding ScrollViewer.VerticalScrollMode}"
                                      ZoomMode="Disabled" />
                        <ContentControl x:Name="PlaceholderTextContentPresenter"
                                        Content="{TemplateBinding PlaceholderText}"
                                        Foreground="{ThemeResource TextBoxPlaceholderTextThemeBrush}"
                                        FontSize="{ThemeResource ContentControlFontSize}"
                                        IsTabStop="False"
                                        Margin="{TemplateBinding BorderThickness}"
                                        Padding="{TemplateBinding Padding}"
                                        Grid.Row="1" />
                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

Furthermore, you can use Blend (tool installed with Visual Studio), to modify controls easier. HERE is the sample tutorial from Microsoft Virtual Academy how to use it with VS2013.