Improve dark mode support

[mwouts]

Closes #294

[github-actions]

Thank you for making this pull request.

Did you know? You can try it on Binder: .

Also, the version of ITables developed in this PR can be installed with pip:

pip install git+https://github.com/mwouts/itables.git@improve_dark_mode

(this requires nodejs, see more at Developing ITables)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.88%. Comparing base (1f6f0a6) to head (ff29305).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #303   +/-   ##
=======================================
  Coverage   95.88%   95.88%           
=======================================
  Files          27       27           
  Lines        1191     1191           
=======================================
  Hits         1142     1142           
  Misses         49       49           

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