[Jetpack] Content Migration App Appearance Fix

[alpavanoglu]

Fixes #19783

To test:

1. Set App Theme on WP to a specific value (not system default)
2. Fresh install on JP
3. Start Migration
4. Verify JP app has the App Theme set from WP when the Migration is complete.

To test:

Regression Notes

1. Potential unintended areas of impact
   N/A

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   N/A

3. What automated tests I added (or what prevented me from doing so)
   N/A

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[twstokes]

This worked for me @alpavanoglu! 🚀

I set WordPress to Dark Mode and when I migrated over to Jetpack it switched to Dark Mode.

[wpmobilebot]

You can test the changes in WordPress from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr19841-69c8f2c on your iPhone

If you need access to App Center, please ask a maintainer to add you.

[wpmobilebot]

You can test the changes in Jetpack from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr19841-69c8f2c on your iPhone

If you need access to App Center, please ask a maintainer to add you.

[spencertransier]

This PR is included in the 21.4.0.3 release.