Remove color override from stepper nav top bar right element

[adamwoodnz]

Part of DSGCOM-92: Update primary buttons on 'Continue as user'

Proposed Changes

• Remove color override from stepper top nav right element
• Extend Akismet branding to cover the stepper nav, as this is the only branded login to use the stepper nav (afaik)

Why are these changes being made?

Overrides to the design system should be avoided, see discussion.

Testing Instructions

• Visit the login page
• Check that the navigation link in the top right corner uses the accent color
• Visit the Akismet login page and do the same.

Screenshots

Before	After

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-145247

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-145247&env=jetpack

Automattic for Agencies live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-145247&env=a8c-for-agencies

[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
• wpcom-block-editor

To test WordPress.com changes, run install-plugin.sh $pluginSlug DSGCOM-92-fix-stepper-nav-link-button-color on your sandbox.

[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.

[adamwoodnz]

@zaerl could you please confirm that this fixes your issue 🙏

[zaerl]

Working as expected. 🚢

[jasmussen]

Thank you for the PR! The dark color is actually intentional as far as the current onboarding designs, though in those designs it should be $gray-900. But this effort, in crossing boundaries across the products, does surface some pushback on that design. To that end, I'd love a broader glance and feedback by @Automattic/dotcom-design before you land this.