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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 
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## 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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