ui: Stepper CSS cleanups from #111036 review feedback

[bogiii]

Part of the follow-up work for the Stepper review feedback in #111036.

Proposed Changes

• Remove the dead color declaration from the base .title rule. It never applied: the stepper root always renders data-indicator-variant, so the higher-specificity [data-indicator-variant] .title rules always win. Addresses this comment.
• Replace the magic margin: 1px on the bullet indicator variant (current and completed states) with calc( ( var( --a8c-ui-stepper-indicator-size ) - var( --a8c-ui-stepper-indicator-size-sm ) ) * 0.5 ), so the centre-alignment compensation is derived from the size variables instead of a hardcoded number. Addresses this comment.

Why are these changes being made?

• Review feedback on feat(ui): add Stepper component to @automattic/ui #111036 flagged a never-applied CSS declaration and a magic number. Removing dead code and deriving the margin from the existing size tokens keeps the styles self-documenting and resilient to future size changes.

Testing Instructions

• Run the Stepper Storybook stories for VerticalStepper and HorizontalStepper with the bullet indicator variant.
• Verify the bullet indicators stay vertically centre-aligned across upcoming, current, and completed states (computed margin should still resolve to 1px: (18px - 16px) / 2).
• Verify step title colors are unchanged in all states (upcoming, current, completed, hover, disabled).

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?
• For UI changes, have you tested the affected components in dark mode?
• Have you tested accessibility for your changes? Ensure the feature remains usable with various user agents (e.g., browsers), interfaces (e.g., keyboard navigation), and assistive technologies (e.g., screen readers) (PCYsg-S3g-p2).
• 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]

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https://calypso.live?image=registry.a8c.com/calypso/app:build-183521&env=jetpack

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Dashboard Live (direct link)

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[matticbot]

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

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

• help-center
• notifications

To test WordPress.com changes, run install-plugin.sh $pluginSlug update/stepper-css-cleanups on your sandbox.

[github-actions]

Looks like one of the E2E tests has failed.

You can fix them following these steps:

1. Check out this branch locally:

   gh pr checkout 111616

2. Start Claude Code in the repo:

   claude

3. Run the /fix-e2e-tests skill, passing this PR number:

   /fix-e2e-tests 111616
   

[ciampo]

LGTM 🚀

Left an optional comment, feel free to address it and merge without the need for another round of review 👍