Fix for secondary window's not respecting app's theme.

[bbonaby]

Summary of the pull request

1. Fixes issue where the secondary windows do not respect the app's theme when its set in the Dev Home settings page. Without this fix the secondary windows continue to keep the Windows system theme, and change when the system theme is changed. For example if Dev Home's theme was set to light theme in the app, but the system theme is dark theme, the secondary window will be in dark theme. Now with this fix, the secondary windows respect the theme set by Dev Home, whether that be light, dark or windows default.
2. Fixes secondary window's titlebar flickering between the Windows default titlebar and our custom titlebar when the window activates.
3. Fixes issue where secondary window is not using a background, and thus appearing to be transparent.

here is a video of how the secondary window works with theme changes after this change.

secondary.window.theme.change.fix.mp4

References and relevant issues

Notice in this video at the end, the last time I close and relaunch the Dev Drive window, the minimize, maximize and close buttons, look like they have a light theme foreground. I checked the Dev Home app itself and it looks like an issue with all the windows of the app currently in Dev builds. So, I opened this issue #1882

Detailed description of the pull request / Additional comments

Validation steps performed

tested this locally.

PR checklist

• Closes #xxx
• Tests added/passed
• Documentation updated

[krschau]

Just curious, would UseAppTheme ever be false (besides in the destructor?)

[dhoehna]

NIT: Consider adding to the comment explaining what needs to be called first.

[AmelBawa-msft]

Just curious, would UseAppTheme ever be false (besides in the destructor?)

@krschau, UseAppTheme is a property I added when creating the SecondaryWindow and is configurable from a custom XAML window. Although it may be uncommon to use it, it simply allows us to create a SecondaryWindow that is theme-independent from the application, and instead follows the system theme.

MyWindow.xaml

<SecondaryWindow
    UseAppTheme="False"
   ....

More details on usage here: SecondaryWindow docs