Add missing styled-system/css external dependency

[jonambas]

What Changed

How To Test or Verify

PR Checklist

• Add the correct type label
• Pull request approval from #uxfe or #design-guild

[codecov]

Codecov Report

Merging #536 into main will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #536   +/-   ##
=======================================
  Coverage   93.29%   93.29%           
=======================================
  Files         118      118           
  Lines        1566     1566           
=======================================
  Hits         1461     1461           
  Misses        105      105           

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[nicklemmon]

@jonambas Looks good!