Donations Block: do not use admin color on the frontend

[jeherve]

Fixes #30811

Proposed changes:

Follow-up from #16687

The Donations block uses a common stylesheet that is used in the block editor and on the frontend, where css vars like --wp-admin-theme-color are not available. Let's switch to using a color that will be available on the frontend.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

• N/A

Does this pull request change what data or activity we track or use?

• No

Testing instructions:

• Start with a site that's connected to WordPress.com.
• Install the Kadence theme
• Go to Posts > Add New and add a Donations block.
• Make sure you can switch between tabs for donation frequency, and that the text is readable.
• Make sure this works on the frontend as well.
• Try with a different theme like Twenty Twenty Three.

[github-actions]

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Jetpack plugin:

• Next scheduled release: June 6, 2023.
• Scheduled code freeze: May 29, 2023.

[github-actions]

Are you an Automattician? You can now test your Pull Request on WordPress.com. On your sandbox, run

bin/jetpack-downloader test jetpack fix/donations-bg-color

to get started. More details: p9dueE-5Nn-p2