fix sidebar.

samples/ControlCatalog/Pages/BorderPage.xaml

@@ -9,7 +9,7 @@
                 Margin="0,16,0,0"
                 HorizontalAlignment="Center"
                 Spacing="16">
-      <Border BorderBrush="{DynamicResource ThemeAccentBrush}" BorderThickness="2" Padding="16">
+      <Border BorderBrush="{DynamicResource SystemAccentColor}" BorderThickness="2" Padding="16">
         <TextBlock>Border</TextBlock>
       </Border>
       <Border Background="{DynamicResource SystemAccentColorDark1}" 

samples/ControlCatalog/SideBar.xaml

@@ -74,7 +74,7 @@
         <Setter Property="Opacity" Value="1"/>
     </Style>
     <Style Selector="TabControl.sidebar > TabItem:pointerover">
-        <Setter Property="Background" Value="Transparent"/>
+        <Setter Property="Background" Value="{DynamicResource SystemAccentColorLight2}"/>
     </Style>
     <Style Selector="TabControl.sidebar > TabItem:selected">
         <Setter Property="Opacity" Value="1"/>