Add ButtonDefault Style, So that users can customize based on the default style.

src/Shared/HandyControl_Shared/Themes/Styles/Button.xaml

@@ -10,19 +10,24 @@
 
     <!--region Button-->
 
-    <Style BasedOn="{StaticResource ButtonBaseStyle}" TargetType="Button">
-        <Setter Property="Foreground" Value="{DynamicResource PrimaryTextBrush}"/>
-        <Setter Property="BorderThickness" Value="1"/>
+    <Style
+        x:Key="ButtonDefault"
+        BasedOn="{StaticResource ButtonBaseStyle}"
+        TargetType="Button">
+        <Setter Property="Foreground" Value="{DynamicResource PrimaryTextBrush}" />
+        <Setter Property="BorderThickness" Value="1" />
         <Style.Triggers>
             <Trigger Property="IsMouseOver" Value="True">
-                <Setter Property="Background" Value="{DynamicResource SecondaryRegionBrush}"/>
+                <Setter Property="Background" Value="{DynamicResource SecondaryRegionBrush}" />
             </Trigger>
             <Trigger Property="IsPressed" Value="True">
-                <Setter Property="Background" Value="{DynamicResource BorderBrush}"/>
+                <Setter Property="Background" Value="{DynamicResource BorderBrush}" />
             </Trigger>
         </Style.Triggers>
     </Style>
 
+    <Style BasedOn="{StaticResource ButtonDefault}" TargetType="Button" />
+
     <Style x:Key="ButtonPrimary" BasedOn="{StaticResource ButtonBaseStyle}" TargetType="Button">
         <Setter Property="Background" Value="{DynamicResource PrimaryBrush}"/>
     </Style>