feat(back-top): use n as CSS vars prefix

[Sepush]

Signed-off-by: Sepush sepush@outlook.com

ref #1842

[vercel]

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[codecov]

Codecov Report

Merging #1893 (31f7ac8) into main (929e5c2) will not change coverage.
The diff coverage is n/a.

❗ Current head 31f7ac8 differs from pull request most recent head 8d42986. Consider uploading reports for the commit 8d42986 to get more accurate results

@@           Coverage Diff           @@
##             main    #1893   +/-   ##
=======================================
  Coverage   64.63%   64.63%           
=======================================
  Files         890      890           
  Lines       17403    17403           
  Branches     4126     4126           
=======================================
  Hits        11248    11248           
  Misses       5375     5375           
  Partials      780      780           

Impacted Files	Coverage Δ
src/back-top/src/BackTop.tsx	62.02% <ø> (ø)
src/back-top/src/styles/index.cssr.ts	100.00% <ø> (ø)

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