Style for Textbox Silverlight

Question

I want to make a simple style for the textbox. I want to retain everything about the standard textbox look and feel except one item.

OnFocus on want to be able to change the border color of the textbox.

I have written the following and it does work. However, everything is restyled, I have to declare height, the look and feel of the non focused border is different as well. How can I create the template to just effect only the onfocus state.

 <Style x:Key="TextBoxStyle" TargetType="TextBox">

            <Setter Property="BorderBrush" Value="Gold" />
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="TextBox">
                        <Grid Height="{TemplateBinding Height}"

                             >
                            <VisualStateManager.VisualStateGroups>
                                <VisualStateGroup x:Name="CommonStates">
                                    <VisualState x:Name="Normal" />
                                    <VisualState x:Name="MouseOver" />
                                    <VisualState x:Name="Pressed" />
                                    <VisualState x:Name="Disabled" />
                                </VisualStateGroup>
                                <VisualStateGroup x:Name="FocusStates">
                                    <VisualState x:Name="Focused">
                                        <Storyboard>
                                            <ColorAnimation Storyboard.TargetName="brd" 
                                                            Storyboard.TargetProperty="(Border.BorderBrush).(SolidColorBrush.Color)" 
                                                            Duration="0" 
                                                            To="Red" />
                                        </Storyboard>
                                    </VisualState>
                                    <VisualState x:Name="Unfocused"/>
                                </VisualStateGroup>
                            </VisualStateManager.VisualStateGroups>
                            <Border x:Name="brd" 
                                    BorderBrush="{TemplateBinding BorderBrush}" 
                                    BorderThickness="{TemplateBinding BorderThickness}" 
                                    Background="{TemplateBinding Background}"
                                    CornerRadius="2">
                                <ContentPresenter x:Name="contentPresenter" />
                            </Border>
                        </Grid>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>

Answer 1

You need to duplicate the entire template of the original TextBox which you can find here. Then make the changes you need.

Answer 2

Example of what SirDemon mentioned...

Here is a style for a textblock:

<Style
    x:Key="detailBlk"
    TargetType="TextBlock">
    <Setter
        Property="FontSize"
        Value="10" />
    <Setter
        Property="Foreground"
        Value="Purple" />
</Style>

Let's say I wanted another style with a FontSize of 20, but the foreground color still purple:

<Style
    x:Key="detailBlk20"
    TargetType="TextBlock"
    BasedOn="{StaticResource detailBlk}">
    <Setter
        Property="FontSize"
        Value="20" />
</Style>

Edit: Sorry, reread question. You want to change the template. The setter properties can set any property. Interestingly, the template is a property so that can be set in the style. However, as far as I know you cannot change individual parts of the template.