Fix #593: Folded code preview should use proper default FG color

[bobbylight]

Fix for #593. The preview tooltips displayed by collapsed folds should follow RSTA's logic, and fall back to the text area's foreground color for any token types without a specific FG color, rather than black.

Before	After

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.50%. Comparing base (54a2d04) to head (798874a).
Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #598      +/-   ##
============================================
+ Coverage     80.49%   80.50%   +0.01%     
+ Complexity     7808     7806       -2     
============================================
  Files           178      178              
  Lines         30263    30253      -10     
  Branches       3914     3912       -2     
============================================
- Hits          24361    24356       -5     
+ Misses         4235     4232       -3     
+ Partials       1667     1665       -2     

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