[a11y] re-enable and fix all failing plan tests

[rukmini-bose]

Addresses #7421 (does not close it)
Closes

Describe your changes:

• enable a11y plan suite
• fix contrast issues that were causing the tests to fail

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Author Checklist

• Changes address original issue?
• Tests included and/or updated with changes?
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Reviewer Checklist

• Changes appear to address issue?
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.64%. Comparing base (a577081) to head (3634ca7).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7753      +/-   ##
==========================================
- Coverage   56.94%   56.64%   -0.31%     
==========================================
  Files         673      673              
  Lines       27177    27177              
  Branches     2636     2636              
==========================================
- Hits        15477    15394      -83     
- Misses      11369    11451      +82     
- Partials      331      332       +1     

Flag	Coverage Δ
e2e-full	23.57% <ø> (-18.51%)	⬇️
e2e-stable	60.63% <ø> (+<0.01%)	⬆️
unit	49.25% <ø> (-0.02%)	⬇️

see 7 files with indirect coverage changes

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