Created attachment 148904 [details]
Screenshot of empty Plasma Styles list, within System Settings.

SUMMARY
The default Plasma styles (Air/Oxygen, Breeze) with their color-scheme adaptive variants are absent from System Settings. However, third-party styles (like Materia) are shown normally.


STEPS TO REPRODUCE
1. Open System Settings
2. Navigate to 'Appearance' -> 'Global Theme' -> 'Plasma Style'
3. Attempt to select a Plasma Style.

OBSERVED RESULT
None of the default styles are shown in the System Settings. Only third-party styles are listed.

EXPECTED RESULT
Both default and third-party styles are shown in System Settings.

SOFTWARE/OS VERSIONS
Operating System: openSUSE Leap 15.3
KDE Plasma Version: 5.24.5
KDE Frameworks Version: 5.94.0
Qt Version: 5.15.2
Kernel Version: 5.3.18-150300.59.63-default (64-bit)
Graphics Platform: X11
Processors: 4 × Intel® Core™ i5-3320M CPU @ 2.60GHz
Memory: 7.6 GiB of RAM
Graphics Processor: Mesa DRI Intel® HD Graphics 4000

ADDITIONAL INFORMATION
I've only seen this bug since the recent KDE Frameworks update to 5.94. However, the issue may have persisted even before then.