Bug#1081109: How to fix

[kritomas]

Can confirm, an update to `kf6-breeze-icons` does fix the issue, although
not entirely. In Dolphin, my desired accent color still renders as black,
the only one to do so. It looks like KDE is caching accent colors, and
cached the broken black. The fix is easy:
1. Switch to any other accent color.
2. Delete `~/.cache`.
3. Reboot.
4. Switch to your old accent color.
This fixes the problem for good.