[#321] Added SCSS var to set a border color for back-to-top component.

[alan-cole]

Closes #321

Checklist before requesting a review

• I have formatted the subject to include the issue number
  as [#123] Verb in past tense with a period at the end.
• I have provided information in the Changed section about WHY something was
  done if this was a bespoke implementation.
• I have performed a self-review of my code.
• I have commented on my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• I have run new and existing relevant tests locally with my changes,
  and they have passed.
• I have provided screenshots, where applicable.

Changed

1. Added SCSS variable to set a border color for back-to-top component.
2. Made the default the background color (same as existing implementation).

Screenshots

[github-actions]

🚀 Deployed on https://668dc9702d4ced16173eeefd--civictheme-uikit.netlify.app

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 13.92%. Comparing base (b81d200) to head (c2a32b6).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #322   +/-   ##
=======================================
  Coverage   13.92%   13.92%           
=======================================
  Files          27       27           
  Lines        1285     1285           
  Branches      306      306           
=======================================
  Hits          179      179           
  Misses        890      890           
  Partials      216      216           

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