[Site Design Revamp] Make the Site Design screen header solid

[twstokes]

Fixes tweak 4 of #18677

This PR matches the top header bar and the background colors of the Site Design screen. If the before photo looks like the after photo don't worry - you probably have normal human vision.

Before	After Light	After Dark

Testing

1. Start the Site Creation flow
2. Navigate to the Site Design screen
3. Observe the background color of the header and the rest of the view

Regression Notes

1. Potential unintended areas of impact

• None

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

• Visually inspected the difference in light and dark modes.

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

• None, as these are visual changes.

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.

[peril-wordpress-mobile]

	Fails
🚫	Podfile: reference to a commit hash

Generated by 🚫 dangerJS

[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 pr18789-727e157 on your iPhone

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

[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 pr18789-727e157 on your iPhone

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

[kyleaparker]

If the before photo looks like the after photo don't worry - you probably have normal human vision.

😆

[antonis]

Being honest I couldn't spot the difference but I validated that the generated build worked on an iPhone SE 2020 (iOS 14.7.1). The code change also look consistent to me 🎉
Thank you for wrangling this @twstokes 🙇

[mkevins]

before photo looks like the after photo

That is indeed very subtle 😄 .