Fix dark theme support for model parameter names

[kecnry]

Description

This pull request fixes dark theme support for model parameter names in the model fitting plugin.

Fixes #1028

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)?

[pllim]

LGTM. Thanks for the quick fix!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.23%. Comparing base (49d6046) to head (ce365f2).
Report is 2772 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1029   +/-   ##
=======================================
  Coverage   73.23%   73.23%           
=======================================
  Files          75       75           
  Lines        5657     5657           
=======================================
  Hits         4143     4143           
  Misses       1514     1514           

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