[Bug 1901431] [NEW] Plain text previews and thumbnails are invisible on dark theme (white on white); please backport the fix

[Geekley]

Public bug reported:

It seems like the fix is available for KDE Applications >= 20.04.
See: https://bugs.kde.org/show_bug.cgi?id=406821
Can you please backport it to Kubuntu 20.04 LTS?

--- Related commits ---
https://phabricator.kde.org/D19432

https://phabricator.kde.org/D21295
https://invent.kde.org/frameworks/syntax-highlighting/commit/bbc465981bd55ca5449f5d22e972fccbe994e24f

https://phabricator.kde.org/D25891
https://invent.kde.org/network/kio-extras/commit/4ec12a850e258de49d2ed7103f82b468939164c1

--- Versions ---
Dolphin 19.12.3
Linux (x86_64) release 5.4.0-52-generic
Qt 5.12.8
KDE Plasma 5.18.5, KDE Frameworks 5.68.0
Plasma Color Scheme: Breeze Dark

-- From KDE Bug Report [+ my additions] ---
SUMMARY
When using a dark UI color theme, with the default syntax highlighting theme for LightTheme those text regions which would be rendered using "Normal" textstyle appear invisible against bright backgrounds.

STEPS TO REPRODUCE
1. Use Plasma, switch to dark color theme "Breeze Dark"
2. In Dolphin, enable preview of "Text Files" (needs KA 19.04)
3. Navigate to folder with text files, e.g. a dir with C++ sources [and .txt files]
4. Enable "Preview" for folder, also trigger tooltips & info docker

OBSERVED RESULT
Rendered text misses out large parts of text to render.
[and plain text files (any file with no highlighting) are invisible: white FG on white BG]

EXPECTED RESULT
All text is properly rendered in some sane colors against the bright background
[or in proper color against dark BG as per the selected theme]

** Affects: dolphin (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1901431

Title:
  Plain text previews and thumbnails are invisible on dark theme (white
  on white); please backport the fix

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