This is caused by the following upstream commit that was in the 6.4.0
release:
https://invent.kde.org/frameworks/breeze-icons/-/commit/931fc4520ceb9ef44f367979d233f2b5bdd4bbb7
modifying icons by using a feature that is not supported/exposed in KF5
kiconthemes.
Possible fixes could be:
1) Reverting upstream commit 931fc4520ce in kf6-breeze-icons. This fixes
the black folder icon issue in KF5 dolphin and application file-pickers,
and a similar issue with some icons in plasma panels/widgets, but
obviously the intention/benefit of the upstream change is lost.
or;
2) Backport KF6
https://invent.kde.org/frameworks/kiconthemes/-/commit/e6fa360255e3a5e9b449be7875904f851601f29e
and
https://invent.kde.org/frameworks/kiconthemes/-/commit/aa89a2ce9d873860f5a858edcd244dfb23241177
KF5 kiconthemes.
These apply cleanly and fix the issue in dolphin and application
file-pickers, but some plasma icons remain affected. Perhaps there is a
further upstream commit that could be backported to fix this. However
once plasma 6 is out of experimental that should no longer be an issue.
or;
(1) in the short term while plasma 6 is still in experimental, switching
to (2) once plasma 6 goes to unstable/testing.
