<Style x:key="NormalStyle" TargetType="{x:Type TextBox}">
    <Setter Property="BorderBrush" Value="Gray" />
    <Setter Property="BorderThickness" Value="2" />
</Style>

<Style x:key="ActiveStyle" TargetType="{x:Type TextBox}" BasedOn="{StaticResource NormalStyle}">
    <Setter Property="BorderBrush" Value="Green" />
    <Setter Property="BorderThickness" Value="4" />
</Style>