-Fixed CodeRenderer again...

-Made Theme Events a Shell Event, that way Theming behaviour can be overidden.

-Added a Theme Picker to the SampleController, with choices of System, Light or Dark for Samples.

-Disabled Background Image Switch from Sample Controller, as Background were too distracting and broke many sample Pages. It could possible be brought back in  a future PR.

Microsoft.Toolkit.Uwp.SampleApp/Controls/CodeRenderer.cs

@@ -13,7 +13,6 @@
 using System;
 using ColorCode;
 using Microsoft.Toolkit.Uwp.Helpers;
-using Microsoft.Toolkit.Uwp.UI.Helpers;
 using Windows.ApplicationModel.DataTransfer;
 using Windows.UI.Popups;
 using Windows.UI.Xaml;
@@ -23,7 +22,6 @@ namespace Microsoft.Toolkit.Uwp.SampleApp.Controls
 {
     public class CodeRenderer : Control
     {
-        private ThemeListener themeListener;
         private RichTextBlockFormatter _formatter;
         private RichTextBlock _codeView;
         private Button _printButton;
@@ -34,13 +32,13 @@ namespace Microsoft.Toolkit.Uwp.SampleApp.Controls
         private string _displayedText;
         private ILanguage _language;
         private bool _rendered;
+        private ElementTheme _theme;
 
         public CodeRenderer()
         {
             DefaultStyleKey = typeof(CodeRenderer);
-
+            _theme = Shell.Current.GetCurrentTheme();
-            themeListener = new ThemeListener();
+            Shell.Current.ThemeChanged += Current_ThemeChanged;
-            themeListener.ThemeChanged += ThemeListener_ThemeChanged;
         }
 
         /// <summary>
@@ -98,13 +96,7 @@ namespace Microsoft.Toolkit.Uwp.SampleApp.Controls
         private void RenderDocument()
         {
             _codeView?.Blocks?.Clear();
-            var theme = themeListener.CurrentTheme == ApplicationTheme.Dark ? ElementTheme.Dark : ElementTheme.Light;
+            _formatter = new RichTextBlockFormatter(_theme);
-            if (RequestedTheme != ElementTheme.Default)
-            {
-                theme = RequestedTheme;
-            }
-
-            _formatter = new RichTextBlockFormatter(theme);
             _formatter.FormatRichTextBlock(_displayedText, _language, _codeView);
             _rendered = true;
         }
@@ -170,8 +162,9 @@ namespace Microsoft.Toolkit.Uwp.SampleApp.Controls
             ReleasePrintHelper();
         }
 
-        private void ThemeListener_ThemeChanged(ThemeListener sender)
+        private void Current_ThemeChanged(object sender, Models.ThemeChangedArgs e)
         {
+            _theme = e.Theme;
             try
             {
                 _rendered = false;

Microsoft.Toolkit.Uwp.SampleApp/Controls/CodeRenderer.xaml

@@ -16,8 +16,13 @@
                         <Grid x:Name="Container"
                                   Grid.RowSpan="2"
                                   Opacity="0" />
-                        <ScrollViewer VerticalScrollBarVisibility="Auto">
+                        <ScrollViewer
-                            <RichTextBlock Name="codeView" />
+                                          Grid.Row="0"
+                                          HorizontalScrollMode="Auto"
+                                          HorizontalScrollBarVisibility="Auto">
+                            <RichTextBlock Name="codeView"
+                                               FontFamily="Consolas"
+                                               Padding="10" />
                         </ScrollViewer>
                         <StackPanel Grid.Row="1"
                                         HorizontalAlignment="Center"

Microsoft.Toolkit.Uwp.SampleApp/Microsoft.Toolkit.Uwp.SampleApp.csproj

@@ -527,6 +527,7 @@
     <Compile Include="Models\LandingPageLink.cs" />
     <Compile Include="Models\PaneState.cs" />
     <Compile Include="Models\PropertyDescriptor\ThicknessPropertyOptions.cs" />
+    <Compile Include="Models\ThemeChangedArgs.cs" />
     <Compile Include="Pages\SampleController.xaml.cs">
       <DependentUpon>SampleController.xaml</DependentUpon>
     </Compile>

Microsoft.Toolkit.Uwp.SampleApp/Models/ThemeChangedArgs.cs

@@ -0,0 +1,18 @@
+using System;
+using Windows.UI.Xaml;
+
+namespace Microsoft.Toolkit.Uwp.SampleApp.Models
+{
+    public class ThemeChangedArgs : EventArgs
+    {
+        /// <summary>
+        /// Gets the Current UI Theme
+        /// </summary>
+        public ElementTheme Theme { get; internal set; }
+
+        /// <summary>
+        /// Gets a value indicating whether the Theme was set by the User.
+        /// </summary>
+        public bool CustomSet { get; internal set; }
+    }
+}

Microsoft.Toolkit.Uwp.SampleApp/Pages/SampleController.xaml

@@ -66,11 +66,22 @@
 
                     <AppBarSeparator Margin="-10, 3, -10, 0" />
 
-                    <ToggleSwitch x:Name="BGSwitch"
+                    <!-- Sample Display properties -->
-                        Margin="10,0,0,0"
+                    <StackPanel Margin="10,0,0,0" Orientation="Horizontal" VerticalAlignment="Center">
+                        <!-- Disabled for Possible Future PR -->
+                        <ToggleSwitch x:Name="BGSwitch"
+                        Visibility="Collapsed"
                         OffContent="BG Image"
                         OnContent="BG Image"
                         IsOn="{x:Bind UseBackground, Mode=TwoWay}" />
+
+                        <TextBlock Text="Theme" VerticalAlignment="Center" Margin="0,0,10,0" />
+                        <ComboBox x:Name="ThemePicker" SelectedIndex="0" SelectionChanged="ThemePicker_SelectionChanged">
+                            <ComboBoxItem Content="System" />
+                            <ComboBoxItem Content="Light" />
+                            <ComboBoxItem Content="Dark" />
+                        </ComboBox>
+                    </StackPanel>
                 </StackPanel>
 
                 <AppBarButton x:Name="NarrowInfoButton"

Microsoft.Toolkit.Uwp.SampleApp/Pages/SampleController.xaml.cs

@@ -21,7 +21,6 @@ using Microsoft.Toolkit.Uwp.SampleApp.Controls;
 using Microsoft.Toolkit.Uwp.SampleApp.Models;
 using Microsoft.Toolkit.Uwp.UI.Controls;
 using Microsoft.Toolkit.Uwp.UI.Extensions;
-using Microsoft.Toolkit.Uwp.UI.Helpers;
 using Windows.System;
 using Windows.System.Profile;
 using Windows.UI.Core;
@@ -91,7 +90,9 @@ namespace Microsoft.Toolkit.Uwp.SampleApp
         {
             this.InitializeComponent();
             Current = this;
+            Shell.Current.ThemeChanged += Current_ThemeChanged;
 
+            DocumentationTextblock.RequestedTheme = Shell.Current.GetCurrentTheme();
             DocumentationTextblock.SetRenderer<SampleAppMarkdownRenderer>();
 
             ProcessSampleEditorTime();
@@ -573,5 +574,18 @@ namespace Microsoft.Toolkit.Uwp.SampleApp
                 VisualStateManager.GoToState(this, NarrowState.Name, false);
             }
         }
+
+        private void Current_ThemeChanged(object sender, ThemeChangedArgs e)
+        {
+            if (e.CustomSet)
+            {
+                DocumentationTextblock.RequestedTheme = e.Theme;
+            }
+        }
+
+        private void ThemePicker_SelectionChanged(object sender, SelectionChangedEventArgs e)
+        {
+            Shell.Current.SetCurrentTheme((ElementTheme)ThemePicker.SelectedIndex);
+        }
     }
 }

Microsoft.Toolkit.Uwp.SampleApp/SamplePages/MarkdownTextBlock/MarkdownTextBlockPage.xaml.cs

@@ -29,20 +29,23 @@ namespace Microsoft.Toolkit.Uwp.SampleApp.SamplePages
     public sealed partial class MarkdownTextBlockPage : Page, IXamlRenderListener
     {
         private TextBox unformattedText;
+        private MarkdownTextBlock markdownText;
 
         public MarkdownTextBlockPage()
         {
             InitializeComponent();
+            Shell.Current.ThemeChanged += Current_ThemeChanged;
         }
 
         public void OnXamlRendered(FrameworkElement control)
         {
             unformattedText = control.FindChildByName("UnformattedText") as TextBox;
 
-            var markdownText = control.FindChildByName("MarkdownText") as MarkdownTextBlock;
+            markdownText = control.FindChildByName("MarkdownText") as MarkdownTextBlock;
 
             if (markdownText != null)
             {
+                markdownText.RequestedTheme = Shell.Current.GetCurrentTheme();
                 markdownText.LinkClicked += MarkdownText_LinkClicked;
                 markdownText.ImageClicked += MarkdownText_ImageClicked;
                 markdownText.CodeBlockResolving += MarkdownText_CodeBlockResolving;
@@ -52,6 +55,14 @@ namespace Microsoft.Toolkit.Uwp.SampleApp.SamplePages
             LoadData();
         }
 
+        private void Current_ThemeChanged(object sender, Models.ThemeChangedArgs e)
+        {
+            if (e.CustomSet)
+            {
+                markdownText.RequestedTheme = e.Theme;
+            }
+        }
+
         private async void MarkdownText_ImageClicked(object sender, LinkClickedEventArgs e)
         {
             if (!Uri.TryCreate(e.Link, UriKind.Absolute, out Uri result))

Microsoft.Toolkit.Uwp.SampleApp/Shell.xaml.cs

@@ -14,9 +14,11 @@ using System;
 using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.Toolkit.Uwp.Helpers;
+using Microsoft.Toolkit.Uwp.SampleApp.Models;
 using Microsoft.Toolkit.Uwp.SampleApp.Pages;
 using Microsoft.Toolkit.Uwp.UI.Controls;
 using Microsoft.Toolkit.Uwp.UI.Extensions;
+using Microsoft.Toolkit.Uwp.UI.Helpers;
 using Windows.Foundation.Metadata;
 using Windows.UI.Core;
 using Windows.UI.Xaml;
@@ -35,6 +37,12 @@ namespace Microsoft.Toolkit.Uwp.SampleApp
             InitializeComponent();
             Current = this;
 
+            _themeListener = new ThemeListener();
+            _themeListener.ThemeChanged += (s) =>
+            {
+                ThemeChanged?.Invoke(this, new ThemeChangedArgs { Theme = GetCurrentTheme() });
+            };
+
             var background = new Image()
             {
                 Source = new BitmapImage(new Uri("ms-appx:///Assets/Photos/Backgrounds/HERO.jpg")),
@@ -72,6 +80,31 @@ namespace Microsoft.Toolkit.Uwp.SampleApp
             }
         }
 
+        /// <summary>
+        /// Gets the Current UI Theme.
+        /// </summary>
+        /// <returns>The Current UI Theme</returns>
+        public ElementTheme GetCurrentTheme()
+        {
+            return RequestedTheme;
+        }
+
+        /// <summary>
+        /// Sets the Current UI Theme.
+        /// </summary>
+        /// <param name="theme">Theme to set</param>
+        public void SetCurrentTheme(ElementTheme theme)
+        {
+            RequestedTheme = theme;
+            var args = new ThemeChangedArgs
+            {
+                CustomSet = true,
+                Theme = GetCurrentTheme()
+            };
+
+            ThemeChanged?.Invoke(this, args);
+        }
+
         public void NavigateToSample(Sample sample)
         {
             NavigationFrame.Navigate(typeof(SampleController), sample);
@@ -198,5 +231,9 @@ namespace Microsoft.Toolkit.Uwp.SampleApp
         }
 
         private UIElement _parallaxView;
+
+        private ThemeListener _themeListener;
+
+        public event EventHandler<ThemeChangedArgs> ThemeChanged;
     }
 }