PR: Fix font color in dark theme for several widgets

[dalthviz]

Pull Request Checklist

• Read and followed this repo's Contributing Guidelines
• Based this PR on the latest version of the correct branch (master or 3.x)
• Followed PEP 8 code style
• Ensured this pull request hasn't eliminated unrelated blank lines/spaces,
  modified the spyder/defaults directory, or added new icons/assets
• Described the changes and the motivation for them below if needed
• Included a screenshot, if this PR makes any visible changes to the UI

Developer Certificate of Origin Affirmation

By submitting this Pull Request or typing my name below, I affirm the
Developer Certificate of Origin
with respect to both the content of the contribution itself and this post,
and understand I am releasing it under Spyder's MIT (Expat) license.

I certify the above statement is true and correct: dalthviz

Description of Changes

Change font color to according to theme in various widgets:

• File switcher
• Symbol finder
• Shortcuts table (preferences)
• LSP table (preferences)
• Find in files results

Issue(s) Resolved

Part of #8068

[pep8speaks]

Hello @dalthviz! Thanks for submitting the PR.

• There are no PEP8 issues in the file spyder/preferences/shortcuts.py !

• There are no PEP8 issues in the file spyder/widgets/fileswitcher.py !

[dalthviz]

@ccordoba12 a preview:

File Switcher

Symbol Finder

Preferences

Find in Files

[ccordoba12]

LGTM, thanks @dalthviz!