fix(List): update styles for Teams Dark and HC themes

[layershifter]

Fixes #1056.

Dark theme

Before

After

HC theme

Before

After

[bmdalex]

👍

[codecov]

Codecov Report

Merging #1117 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1117      +/-   ##
==========================================
+ Coverage   82.02%   82.03%   +<.01%     
==========================================
  Files         720      722       +2     
  Lines        8605     8609       +4     
  Branches     1234     1234              
==========================================
+ Hits         7058     7062       +4     
  Misses       1530     1530              
  Partials       17       17

Impacted Files	Coverage Δ
.../themes/teams/components/List/listItemVariables.ts	66.66% <ø> (ø)	⬆️
...es/teams-dark/components/List/listItemVariables.ts	100% <100%> (ø)
.../react/src/themes/teams-dark/componentVariables.ts	100% <100%> (ø)	⬆️
...c/themes/teams-high-contrast/componentVariables.ts	100% <100%> (ø)	⬆️
...high-contrast/components/List/listItemVariables.ts	100% <100%> (ø)

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