fix: make sure custom colours don't override newsletter modern style

[laurelfulford]

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:

This PR fixes a contrast issue with the theme's checkbox styles, and the Newsletter Subscription Block's modern style -- the theme typically uses the secondary colour for the checkbox background, but the modern style always uses black. When you pick a light secondary colour, the checkmark ends up being black, which can't be seen against the black background.

See 1200550061930446-as-1208925182714392

How to test the changes in this Pull Request:

1. Under Customizer > Custom Colours, change your secondary colour to something light.
2. Add a Newsletter Subscription block to a post or page, and enable at least two lists.
3. On the front end, test checking one of the lists (if they're not pre-selected), and note that you can't see the checkmark:

4. Apply the PR and run npm run build.
5. Refresh the page; confirm that the checkboxes look okay:

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?

[dkoo]

Works!

[laurelfulford]

Thanks @dkoo!

[github-actions]

Hey @laurelfulford, good job getting this PR merged! 🎉

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Thank you! ❤️

[matticbot]

🎉 This PR is included in version 2.4.3-alpha.1 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀

[matticbot]

🎉 This PR is included in version 2.4.3 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀