build: add bg-mpl-stylesheet to have better plot

[stevenhua0320]

@sbillinge ready to review, now with the bg-mpl-stysheet included, we could have better color effect on the plot. But we might need to release a new version of bg-mpl-stylesheet to python 3.14. Closes #146

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.47%. Comparing base (d9e435d) to head (70bbcf4).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #180   +/-   ##
=======================================
  Coverage   76.47%   76.47%           
=======================================
  Files           3        3           
  Lines          51       51           
=======================================
  Hits           39       39           
  Misses         12       12           

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