Search bar: update border color when dark/light mode toggled.

[ScoutHarris]

Fixes #13364

This fixes the border color of the Search bar when light/dark mode toggled.

To test:

• Go to any view with a Search bar. (ex: Site Pages, Blog Posts, Themes)
• Switch between light and dark modes.
• Verify the Search bar border color is updated accordingly.

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger an installable build for these changes by visiting CircleCI here.

[mindgraffiti]

Ahhh thank you. That border was really noticeable 😀

✅ code
✅ UI tested