How to extend instead of overriding WPF Styles

I had the same problem. I used Zack’s answer and improved it like following so if you don’t specify a style the overridden default is still taken in account. It’s basically what you would have done but just once in the ResourceDictionary.

<Window x:Class="TestWpf.RandomStuffWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Random Stuff Window">
  <Window.Resources>
    <ResourceDictionary>
      <!-- Default Label style definition -->
      <Style TargetType="{x:Type Label}">
        <Setter Property="Foreground" Value="Green" />
      </Style>
      <!-- Extending default style -->
      <Style TargetType="{x:Type Label}" 
             x:Key="LargeGreenForegroundLabel" 
             BasedOn="{StaticResource {x:Type Label}}">
        <Setter Property="FontSize" Value="28" />
      </Style>
    </ResourceDictionary>
  </Window.Resources>
  <StackPanel>
    <Button Click="Button_Click">Click</Button>
    <Label Content="GreenForegroundLabel" /> <!-- Uses default style -->
    <Label Style="{StaticResource LargeGreenForegroundLabel}" 
           Content="LargeGreenForegroundLabel" />
  </StackPanel>
</Window>