fixed scrollviewerextensions sample on rs1

Author: nmetulev

Microsoft.Toolkit.Uwp.SampleApp/SamplePages/ScrollViewerExtensions/ScrollViewerExtensionsCode.bind

@@ -18,7 +18,7 @@
         <controls:ScrollHeader Mode="Sticky">
           <Grid x:Name="MyHeaderGrid"
                 MinHeight="@[Header MinHeight:DoubleSlider:250:150-500]"
-                Background="{ThemeResource SystemControlAccentAcrylicElementAccentMediumHighBrush}">
+                Background="#AA000000">
             <StackPanel HorizontalAlignment="Center"
                         VerticalAlignment="Center">
               <TextBlock Margin="12"

Microsoft.Toolkit.Uwp.SampleApp/SamplePages/ScrollViewerExtensions/ScrollViewerExtensionsPage.xaml

@@ -19,7 +19,7 @@
                 <controls:ScrollHeader Mode="Sticky">
                     <Grid x:Name="MyHeaderGrid"
                           MinHeight="100"
-                          Background="{ThemeResource SystemControlAccentAcrylicElementAccentMediumHighBrush}">
+                          Background="#AA000000">
                         <StackPanel HorizontalAlignment="Center"
                                     VerticalAlignment="Center">
                             <TextBlock Margin="12"

readme.md

@@ -113,6 +113,7 @@ Once you search you should see a list similar to the one below (versions may be
 * [LogicalTree](https://docs.microsoft.com/windows/uwpcommunitytoolkit/extensions/LogicalTree/)
 * [MouseCursor](https://docs.microsoft.com/windows/uwpcommunitytoolkit/extensions/MouseCursor/)
 * [NavigationView Styles](https://docs.uwpcommunitytoolkit.com/windows/uwpcommunitytoolkit/extensions/NavigationView/)
+* [ScrollViewerExtensions](https://docs.microsoft.com/windows/uwpcommunitytoolkit/extensions/ScrollViewerExtensions/)
 * [SurfaceDialTextbox](https://docs.microsoft.com/windows/uwpcommunitytoolkit/extensions/SurfaceDialTextboxHelper/)
 * [TextBoxMask](https://docs.microsoft.com/windows/uwpcommunitytoolkit/extensions/TextBoxMask/)
 * [TextBoxRegex](https://docs.microsoft.com/windows/uwpcommunitytoolkit/extensions/TextBoxRegex/)