Reader: Fix back button styles on logged out Discover signup flow

[artemiomorales]

Related to #101584

Proposed Changes

• Adds styles to override the Reader back button stylesheet

Before

After

Why are these changes being made?

• The back button styles for the signup page are broken when going through the /discover flow because the Reader's styles are being included

Testing Instructions

• Go to /discover while logged out
• On the sign up screen, click 'Email'
• Make sure the back button appears below the email input field as expected

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-138394

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-138394&env=jetpack

Automattic for Agencies live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-138394&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.

[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/sign-up-back-button-styles on your sandbox.

[DustyReagan]

Looks good! I tried signing up from /discover, /setup/domain-transfer/, and /setup/newsletter, and I don't see any regressions. ✅