Add styles to theme for completion item and metadata colors

[tleonhardt]

Add styles to theme for completion item and metadata foreground and background colors.

This is an attempt to provide higher-contrast color options and to make them configurable as part of a theme.

[codecov]

Codecov Report

❌ Patch coverage is 71.05263% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.26%. Comparing base (12a7d5e) to head (1a73c22).

Files with missing lines	Patch %	Lines
cmd2/cmd2.py	31.25%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1653      +/-   ##
==========================================
- Coverage   99.49%   99.26%   -0.23%     
==========================================
  Files          21       21              
  Lines        4747     4785      +38     
==========================================
+ Hits         4723     4750      +27     
- Misses         24       35      +11     

Flag	Coverage Δ
unittests	99.26% <71.05%> (-0.23%)	⬇️

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