Added theme-specific background for XAML code errors (#3588)

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## Fixes #3545 
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<!-- Add a brief overview here of the feature/bug & fix. -->
Adds a theme listener field and a read only property that returns the theme-specific error background color to the sample app's Xaml code editor.

## PR Type
What kind of change does this PR introduce?
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Bugfix
<!-- - Feature -->
<!-- - Code style update (formatting) -->
<!-- - Refactoring (no functional changes, no api changes) -->
<!-- - Build or CI related changes -->
<!-- - Documentation content changes -->
Sample app changes 
<!-- - Other... Please describe: -->

## What is the current behavior?
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Quoting from #3545 
> Errors in the XAML Sample are hard to read as the background-red does not work well with text:
> ![](https://user-images.githubusercontent.com/16122379/97060991-92139780-1595-11eb-9e26-a9e81e28dda8.png)

## What is the new behavior?
<!-- Describe how was this issue resolved or changed? -->
A different text background is now displayed when dark theme is enabled, making it more readable.
![image](https://user-images.githubusercontent.com/59544401/100564314-8f335300-3275-11eb-8fe0-68b1ac91c6d3.png)

## PR Checklist

Please check if your PR fulfills the following requirements:

 - [x] Tested code with current supported SDKs
 - [N/A] Pull Request has been submitted to the documentation repository instructions. Link:
 - [x] Sample in sample app has been added / updated (for bug fixes / features)
      - [N/A] Icon has been created (if new sample) following the Thumbnail Style Guide and templates
 - [N/A] Tests for the changes have been added (for bug fixes / features) (if applicable)
 - [N/A] Header has been added to all new source files (run build/UpdateHeaders.bat) (None added)
 - [x] Contains NO breaking changes

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Author: msftbot[bot]

Microsoft.Toolkit.Uwp.SampleApp/Controls/XamlCodeEditor.xaml.cs

@@ -21,6 +21,8 @@ public sealed partial class XamlCodeEditor : UserControl
         public static readonly DependencyProperty TextProperty =
             DependencyProperty.Register(nameof(Text), typeof(string), typeof(XamlCodeEditor), new PropertyMetadata(string.Empty));
 
+        private ThemeListener _themeListener = new ThemeListener();
+
         public XamlCodeEditor()
         {
             this.InitializeComponent();
@@ -40,7 +42,7 @@ public async void ReportError(XamlExceptionRange error)
             // Highlight Error Line
             XamlCodeRenderer.Decorations.Add(new IModelDeltaDecoration(
                 range,
-                new IModelDecorationOptions() { IsWholeLine = true, ClassName = _errorStyle, HoverMessage = new string[] { error.Message }.ToMarkdownString() }));
+                new IModelDecorationOptions() { IsWholeLine = true, ClassName = ErrorStyle, HoverMessage = new string[] { error.Message }.ToMarkdownString() }));
 
             // Show Glyph Icon
             XamlCodeRenderer.Decorations.Add(new IModelDeltaDecoration(
@@ -121,10 +123,12 @@ public string Text
 
         public DateTime TimeSampleEditedLast { get; private set; } = DateTime.MinValue;
 
-        private CssLineStyle _errorStyle = new CssLineStyle()
+        private CssLineStyle ErrorStyle
         {
-            BackgroundColor = new SolidColorBrush(Color.FromArgb(0x00, 0xFF, 0xD6, 0xD6))
-        };
+            get => _themeListener.CurrentTheme.Equals(ApplicationTheme.Light) ?
+                new CssLineStyle() { BackgroundColor = new SolidColorBrush(Color.FromArgb(0x00, 0xFF, 0xD6, 0xD6)) } :
+                new CssLineStyle() { BackgroundColor = new SolidColorBrush(Color.FromArgb(0x00, 0x66, 0x00, 0x00)) };
+        }
 
         private CssGlyphStyle _errorIconStyle = new CssGlyphStyle()
         {