How to change the color/opacity of the triangular icon on a TreeViewItem?

Answer 1

You will have to edit the ControlTemplate. I extracted the default one for you with Expression Blend. You need to muck around with it until you achieve the effect you want and then apply it to your TreeViewItems like this:

  <TreeViewItem Header="Word 1" ItemContainerStyle="{DynamicResource TreeViewItemStyle1}"/>

Here is the style itself:

        <Style x:Key="TreeViewItemFocusVisual">
         <Setter Property="Control.Template">
          <Setter.Value>
           <ControlTemplate>
            <Rectangle/>
           </ControlTemplate>
          </Setter.Value>
         </Setter>
        </Style>
        <PathGeometry x:Key="TreeArrow" Figures="M0,0 L0,6 L6,0 z"/>
        <Style x:Key="ExpandCollapseToggleStyle" TargetType="{x:Type ToggleButton}">
         <Setter Property="Focusable" Value="False"/>
         <Setter Property="Width" Value="16"/>
         <Setter Property="Height" Value="16"/>
         <Setter Property="Template">
          <Setter.Value>
           <ControlTemplate TargetType="{x:Type ToggleButton}">
            <Border Width="16" Height="16" Background="Transparent" Padding="5,5,5,5">
             <Path x:Name="ExpandPath" Fill="Transparent" Stroke="#FF989898" Data="{StaticResource TreeArrow}">
              <Path.RenderTransform>
               <RotateTransform Angle="135" CenterX="3" CenterY="3"/>
              </Path.RenderTransform>
             </Path>
            </Border>
            <ControlTemplate.Triggers>
             <Trigger Property="IsMouseOver" Value="True">
              <Setter Property="Stroke" TargetName="ExpandPath" Value="#FF1BBBFA"/>
              <Setter Property="Fill" TargetName="ExpandPath" Value="Transparent"/>
             </Trigger>
             <Trigger Property="IsChecked" Value="True">
              <Setter Property="RenderTransform" TargetName="ExpandPath">
               <Setter.Value>
                <RotateTransform Angle="180" CenterX="3" CenterY="3"/>
               </Setter.Value>
              </Setter>
              <Setter Property="Fill" TargetName="ExpandPath" Value="#FF595959"/>
              <Setter Property="Stroke" TargetName="ExpandPath" Value="#FF262626"/>
             </Trigger>
            </ControlTemplate.Triggers>
           </ControlTemplate>
          </Setter.Value>
         </Setter>
        </Style>
        <Style x:Key="TreeViewItemStyle1" TargetType="{x:Type TreeViewItem}">
         <Setter Property="Background" Value="Transparent"/>
         <Setter Property="HorizontalContentAlignment" Value="{Binding HorizontalContentAlignment, RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}"/>
         <Setter Property="VerticalContentAlignment" Value="{Binding VerticalContentAlignment, RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}"/>
         <Setter Property="Padding" Value="1,0,0,0"/>
         <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
         <Setter Property="FocusVisualStyle" Value="{StaticResource TreeViewItemFocusVisual}"/>
         <Setter Property="Template">
          <Setter.Value>
           <ControlTemplate TargetType="{x:Type TreeViewItem}">
            <Grid>
             <Grid.ColumnDefinitions>
              <ColumnDefinition MinWidth="19" Width="Auto"/>
              <ColumnDefinition Width="Auto"/>
              <ColumnDefinition Width="*"/>
             </Grid.ColumnDefinitions>
             <Grid.RowDefinitions>
              <RowDefinition Height="Auto"/>
              <RowDefinition/>
             </Grid.RowDefinitions>
             <ToggleButton x:Name="Expander" Style="{StaticResource ExpandCollapseToggleStyle}" ClickMode="Press" IsChecked="{Binding IsExpanded, RelativeSource={RelativeSource TemplatedParent}}"/>
             <Border x:Name="Bd" SnapsToDevicePixels="true" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Padding="{TemplateBinding Padding}" Grid.Column="1">
              <ContentPresenter x:Name="PART_Header" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" ContentSource="Header"/>
             </Border>
             <ItemsPresenter x:Name="ItemsHost" Grid.Column="1" Grid.ColumnSpan="2" Grid.Row="1"/>
            </Grid>
            <ControlTemplate.Triggers>
             <Trigger Property="IsExpanded" Value="false">
              <Setter Property="Visibility" TargetName="ItemsHost" Value="Collapsed"/>
             </Trigger>
             <Trigger Property="HasItems" Value="false">
              <Setter Property="Visibility" TargetName="Expander" Value="Hidden"/>
             </Trigger>
             <Trigger Property="IsSelected" Value="true">
              <Setter Property="Background" TargetName="Bd" Value="{DynamicResource {x:Static SystemColors.HighlightBrushKey}}"/>
              <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.HighlightTextBrushKey}}"/>
             </Trigger>
             <MultiTrigger>
              <MultiTrigger.Conditions>
               <Condition Property="IsSelected" Value="true"/>
               <Condition Property="IsSelectionActive" Value="false"/>
              </MultiTrigger.Conditions>
              <Setter Property="Background" TargetName="Bd" Value="{DynamicResource {x:Static SystemColors.ControlBrushKey}}"/>
              <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
             </MultiTrigger>
             <Trigger Property="IsEnabled" Value="false">
              <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
             </Trigger>
            </ControlTemplate.Triggers>
           </ControlTemplate>
          </Setter.Value>
         </Setter>
         <Style.Triggers>
          <Trigger Property="VirtualizingStackPanel.IsVirtualizing" Value="true">
           <Setter Property="ItemsPanel">
            <Setter.Value>
             <ItemsPanelTemplate>
              <VirtualizingStackPanel/>
             </ItemsPanelTemplate>
            </Setter.Value>
           </Setter>
          </Trigger>
         </Style.Triggers>
        </Style>