update Expander & GroupBox styles

src/Shared/HandyControl_Shared/Themes/Styles/Base/PropertyGridBaseStyle.xaml

@@ -114,7 +114,7 @@
         <Setter Property="Template">
             <Setter.Value>
                 <ControlTemplate TargetType="GroupItem">
-                    <Expander BorderThickness="1" BorderBrush="{DynamicResource BorderBrush}" Header="{Binding Name}" IsExpanded="True" Background="{DynamicResource SecondaryRegionBrush}" Foreground="{DynamicResource PrimaryTextBrush}">
+                    <Expander Header="{Binding Name}" IsExpanded="True">
                         <Border BorderThickness="1,0,1,1" BorderBrush="{DynamicResource BorderBrush}" Background="{DynamicResource RegionBrush}" CornerRadius="0,0,4,4">
                             <ItemsPresenter Margin="{TemplateBinding Padding}"/>
                         </Border>
@@ -163,4 +163,4 @@
         </Setter>
     </Style>
 
-</ResourceDictionary>
+</ResourceDictionary>