Add example for setting a theme

[tleonhardt]

This is an attempt to add an example demonstrating how to create a custom theme for a cmd2 application.

It currently overrides a few elements of the theme. We should override more elements to make things more discoverable for users.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.78%. Comparing base (b9c6369) to head (2125b00).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1497      +/-   ##
==========================================
+ Coverage   98.76%   98.78%   +0.02%     
==========================================
  Files          23       23              
  Lines        4925     4925              
==========================================
+ Hits         4864     4865       +1     
+ Misses         61       60       -1     

Flag	Coverage Δ
unittests	98.78% <ø> (+0.02%)	⬆️

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