Fix default color for separator component

[tom2drum]

Description and Related Issue(s)

This PR standardizes the separator component's border color by introducing a default theme recipe and removing redundant borderColor props across the codebase.

Proposed Changes

• Added a new separator.recipe.ts that sets borderColor: 'border.divider' as the default for all separator components
• Removed explicit borderColor="border.divider" props from separator components in:
  • BlockDetailsZilliqaQuorumCertificate.tsx
  • Content.tsx (revoke component)
  • RewardsStreakModal.tsx
  • TopBar.tsx
• Updated NavLinkGroup.tsx to use alignItems="stretch" for better separator alignment

This change centralizes separator styling in the theme recipe, making it easier to maintain and ensuring consistent appearance across the application.

Breaking or Incompatible Changes

None.

Additional Information

N/A

Checklist for PR author

• I have tested these changes locally.
• I added tests to cover any new functionality, following this guide
• Whenever I fix a bug, I include a regression test to ensure that the bug does not reappear silently.
• If I have added, changed, renamed, or removed an environment variable
  • I updated the list of environment variables in the documentation
  • I made the necessary changes to the validator script according to the guide
  • I added "ENVs" label to this pull request