Parts of the application are unreadable when changing from light mode to dark mode

[Yajur-Grover]

When switching from light mode to dark mode, parts of the NewArch Gallery application become quite unreadable as shown below:

Light mode:

Dark mode:

[chrisglein]

Given the Paper version of the app uses PlatformColor throughout and works on light/dark mode. However on Fabric this no longer can directly map to XAML resource dictionary values (only the core standard Windows colors). Also this should be reconciled alongside a macOS version of the app (hi @Saadnajmi). Also also need to document the difference in behavior on Fabric vs. Paper here because other apps will see migration issues if they have deep PlatformColor usage (XAML color constants specifically).

@Yajur-Grover Can you check the color usage in the app to see where it's using XAML-based colors (shouldn't work) vs. potential gaps in our Fabric dictionary. And also add to the Fabric migration page in the documentation about this issue (create a new issue to track that, separate from fixing the gallery repo behavior). It's a known breaking change... but known to us not known in the docs :D