Settings: Fix oobe page markdown theme color for H1 and H2

[vanzue]

Summary of the Pull Request

How to repro:
change theme in settings, observe the oobe what's new's markdown render

PR Checklist

• Closes: Wrong title color in what's new #45713

• Communication: I've discussed this with core contributors already. If the work hasn't been agreed, this work might be rejected
• Tests: Added/updated and all pass
• Localization: All end-user-facing strings can be localized
• Dev docs: Added/updated
• New binaries: Added on the required places
  • JSON for signing for new binaries
  • WXS for installer for new binaries and localization folder
  • YML for CI pipeline for new test projects
  • YML for signed pipeline
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed

1. Light and dark mode works as usual
2. When switch from light to dark, dark to light, everything is colored correctly

fix-oobe-theme.mp4

[niels9001]

@vanzue We should fix this upstream in the MarkdownTextBlock, because this will hit us in other areas too (like CmdPal). I've asked the Toolkit team to look at it: CommunityToolkit/Labs-Windows#785

[vanzue]

Great to know!