Forms: Fixes date picker field compatibility with dark themes

[enejb]

Fixes #29793

Before:

After:

Proposed changes:

• Add colour and underline styling for the datepicker so that the picker renders more as expected.

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?
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Jetpack product discussion

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

Testing instructions:

• Load up this PR.
• Set the twenty twenty five theme to pallet dark mode
• Add the appointment form to a page.
• Notice that the date picker now renders the header and the month navigation.

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WordPress.com Simple site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin, and enable the fix/editor-select-field branch.

• To test on Simple, run the following command on your sandbox:

  bin/jetpack-downloader test jetpack fix/editor-select-field
  

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[github-actions]

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[edanzer]

In case anyone else wonders... I was initially concerned about hard-coding a css value for font color like #444. For most of our form block styling, we try to inherit theme styles and use Jetpack CSS variables that are dynamically calculated. I wondered if a user might set a field background color to be dark, which would cause issues with the hard-coded text color.

But from what I can see, the calendar background is always white. Updating field background colors does not affect the calendar.

And, to emphasize the point, we already hardcode a default #444 text color for the calendar here.