A4A: adds 'Dark' mode support on the new Signup form.

[jkguidaven]

This PR adds 'Dark' mode support on the new Signup form.

Screen.Recording.2025-02-07.at.12.20.11.AM.mov

Depends on #99414
Closes https://github.com/Automattic/automattic-for-agencies-dev/issues/1740

Proposed Changes

• Add 'Dark' preference CSS rules.

Why are these changes being made?

This is part of the new Signup form project for WC Asia preparation.

Testing Instructions

• Set your browser preference to 'Dark' mode.
• Use the A4A live link and go to the /signup/wc-asia page.
• Test that the form 'Dark' mode styling follows the design

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
  • For UI changes, have we tested the change in various languages (for example, ES, PT, FR, or DE)? The length of text and words vary significantly between languages.
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-132430

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-132430&env=jetpack

Automattic for Agencies live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-132430&env=a8c-for-agencies

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[andrii-lysenko]

A small nitpick regarding Country selector but looks great otherwise

[jkguidaven]

A small nitpick regarding Country selector but looks great otherwise

I tried to style it, but it was complicated to override with the CSS rule because it is a styled component (randomized internal class names). We can address this in a separate PR so we don't get blocked.

[jkguidaven]

For some reason, lint is complaining now.

[matticbot]

This PR modifies the release build for the following Calypso Apps:

For info about this notification, see here: PCYsg-OT6-p2

• notifications
• wpcom-block-editor

To test WordPress.com changes, run install-plugin.sh $pluginSlug fix/a4a/signup-form-v2-dark-mode on your sandbox.