Set default BorderThickness to 0 to avoid white borders around every Expander

Author: chrischu

MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.Expander.xaml

@@ -219,6 +219,7 @@
     </Style>
 
     <Style x:Key="MaterialDesignExpander" TargetType="{x:Type Expander}">
+        <Setter Property="BorderThickness" Value="0" />
         <Setter Property="Background" Value="{DynamicResource MaterialDesignPaper}" />
         <Setter Property="Foreground" Value="{Binding RelativeSource={RelativeSource AncestorType={x:Type FrameworkElement}}, Path=(TextElement.Foreground)}"/>
         <Setter Property="HorizontalContentAlignment" Value="Stretch"/>