fix: use hex value for group background colors in default layouts

[jeffersonrabb]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Fixes an additional issue with Group Block background color. Three of the four default layouts use very-light-gray as their background. This color option is no longer available, so the blocks have no background color when rendered to email markup. This PR replaces them with the hex color #f0f0f0.

How to test the changes in this Pull Request:

1. On master, create Newsletters with the "Breaking News," "Daily/Weekly (no image)," or "Daily/Weekly" default layouts.
2. Send a test email, observe the Group block backgrounds are not present in the email.
3. Switch to this branch and repeat.
4. Observe the backgrounds are present in the email.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[matticbot]

🎉 This PR is included in version 1.8.1 🎉

The release is available on GitHub release

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