How to change the appearance of volume & brightness overlay in dark mode?
I tried to use bundle id com.apple.OSDUIHelper to exclude OSD from the aqua-appearance fixup, but with no luck – like it ignores per-app defaults.
However I've got what I wanted with following steps:

killall -9 OSDUIHelper; defaults write -g NSRequiresAquaSystemAppearance -bool NO
Perform some action to show OSD, so it will started in normal configuration
defaults write -g NSRequiresAquaSystemAppearance -bool YES

After that OSD becomes normal until reboot.
I didn't find any way to trigger OSD from a shell script to automate these steps, so I made the app:
https://github.com/dstd/osdui-aqua-fix/releases/tag/v1