Added more options to ZefyrThemeData.

[xxdocobxx]

• Added the parameter horizontalRuleColor to ZefyrThemeData.
• Added the parameter backgroundColor to quote and code BlockTheme.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.22%. Comparing base (86c4dd0) to head (f63fe3a).
Report is 222 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #107      +/-   ##
==========================================
+ Coverage   92.11%   93.22%   +1.11%     
==========================================
  Files          12       12              
  Lines        1090     1077      -13     
==========================================
  Hits         1004     1004              
+ Misses         86       73      -13     

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