Changing styles when text has been changed

Having trouble figuring how out to have the text change styles when the text is changed. I have it set up so the styles change when I am typing and the default is set to be grayed out to indicate what needs to go in that field. I want the text to be black and of normal font weight when it is changed from the default text and to remain that way.

Here are my current trigger styles

<Style TargetType="{x:Type TextBox}">
        <Setter Property="Foreground" Value="DimGray" />
        <Setter Property="FontSize" Value="15" />
        <Setter Property="FontStyle" Value="Italic" />
        <Setter Property="Padding" Value="5,4,0,4" />
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type TextBoxBase}">
                    <Border
        x:Name="Bd" 
        BorderBrush="{TemplateBinding BorderBrush}" 
        BorderThickness="{TemplateBinding BorderThickness}"
        CornerRadius="7"
        >
                        <Border.Background>
                            <LinearGradientBrush StartPoint="0,0" EndPoint="2,1">
                                <GradientStop Color="{Binding Path=GradientColorStart}" Offset="0"/>
                                <GradientStop Color="{Binding Path=GradientColorEnd}" Offset="1"/>
                                <!-- 
                                <GradientStop Color="#CCCCCC" Offset="0"/>
                      开发者_StackOverflow          <GradientStop Color="#FFFFFF" Offset="1"/>
                                -->
                            </LinearGradientBrush>
                        </Border.Background>
                        <ScrollViewer x:Name="PART_ContentHost"/>
                    </Border>
                    <ControlTemplate.Triggers>
                        <Trigger Property="IsEnabled" Value="False">
                            <Setter 
                Property="Background" 
                TargetName="Bd"
                Value="{DynamicResource {x:Static SystemColors.ControlBrushKey}}" 
                />
                            <Setter Property="Foreground" Value="Green"/>
                        </Trigger>
                        <Trigger Property="Width" Value="Auto">
                            <Setter Property="MinWidth" Value="100"/>
                        </Trigger>
                        <Trigger Property="Height" Value="Auto">
                            <Setter Property="MinHeight" Value="20"/>
                        </Trigger>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
        <Style.Triggers>
            <Trigger Property="IsFocused" Value="True">
                <Setter Property="Foreground" Value="Black" />
                <Setter Property="FontWeight" Value="Bold" />
                <Setter Property="FontStyle" Value="Normal" />
                <Setter Property="Text" Value="" />
            </Trigger>
            <Trigger Property="IsEnabled" Value="False">
                <Setter Property="Foreground" Value="#00112d" />
                <Setter Property="FontWeight" Value="Normal" />
                <Setter Property="FontStyle" Value="Normal" />
            </Trigger>
        </Style.Triggers>
    </Style>

This is something i would approach with the TargetNullValue of bindings, the string has to be null at first of course, so this might not be an option for you:

<TextBox Text="{Binding TestString, TargetNullValue=Enter Text Here, UpdateSourceTrigger=PropertyChanged}">
    <TextBox.Style>
        <Style TargetType="{x:Type TextBox}">
            <Style.Triggers>
                <DataTrigger Binding="{Binding TestString}" Value="{x:Null}">
                    <Setter Property="Foreground" Value="{StaticResource {x:Static SystemColors.GrayTextBrushKey}}"/>
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </TextBox.Style>
</TextBox>