fix plugin description text color in dark theme

[kecnry]

Description

This pull request fixes the text color of the plugin descriptions to be dark-theme-friendly.

Fixes #933

Checklist for package maintainer(s)

This checklist is meant to remind the package maintainer(s) who will review this pull request of some common things to look for. This list is not exhaustive.

• Are two approvals required? Branch protection rule does not check for the second approval. If a second approval is not necessary, please apply the trivial label.
• Do the proposed changes actually accomplish desired goals? Also manually run the affected example notebooks, if necessary.
• Do the proposed changes follow the STScI Style Guides?
• Are tests added/updated as required? If so, do they follow the STScI Style Guides?
• Are docs added/updated as required? If so, do they follow the STScI Style Guides?
• Did the CI pass? If not, are the failures related?
• Is a change log needed? If yes, is it added to CHANGES.rst?
• Is a milestone set? Milestone is only currently required for PRs related to Imviz MVP.
• After merge, any internal documentations need updating (e.g., JIRA, Innerspace)?

[codecov]

Codecov Report

Merging #937 (91967bc) into main (4c2c3a5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #937   +/-   ##
=======================================
  Coverage   69.37%   69.37%           
=======================================
  Files          69       69           
  Lines        4888     4888           
=======================================
  Hits         3391     3391           
  Misses       1497     1497           

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[pllim]

LGTM. Thanks! Maybe another dev wanna double check on a different setting/machine as well.

[ojustino]

This change works for me, too.