Improve contrast ratio in annotation textarea

[acelaya]

As part of #6866, and following #6866 (comment), this PR updates the text color and placeholder color in the annotation textarea to darker shades of grey:

• grey-6 (#737373) for placeholder.

  I originally recommended #767676, but the difference is hard to notice, so using grey-6 we avoid having to define a new color.

• grey-9 (#202020) for the text, via inheriting the page color, as the rest of the inputs do.

This PR does not cover changing this anywhere other than the annotation textarea. I'll prepare follow-up PRs to update the defaults in the shared library and other projects.

Before:

After:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.44%. Comparing base (a1f38f3) to head (bc57280).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6966   +/-   ##
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  Coverage   99.44%   99.44%           
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  Files         275      275           
  Lines       10906    10906           
  Branches     2631     2631           
=======================================
  Hits        10846    10846           
  Misses         60       60           

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[robertknight]

Slack discussion

[robertknight]

This direction looks good to me, assuming we roll out a consistent change in our component library.

[acelaya]

This direction looks good to me, assuming we roll out a consistent change in our component library.

I'll wait to merge this to have the change in the shared library ready. That way we can roll both changes out at the same time.

[acelaya]

Inherit default text color (grey-9) and set a dark-enough placeholder color that has enough contrast with white backgrounds.