However, somehow they are not saved and in the next session GTK styles will be corrupted again.
Sound volume levels are also not preserved between sessions and visual feedback is inconsistent, e.g.: in the following image you can see that the volume bar is at 100% while actually the volume is at 5%.
I actually found that commenting the line
in ~/.config/gtk-3.0/settings.ini also works, but this file is overwritten and reset to the old state at each login, which can be solved by changing the owner to root. What a hack, that is.
I just wonder what happens if plasma is updated again.
I went to KDE Settings > Application Style > Configure GNOME/GTK. The single dropdown box was empty. I made a selection and clicked Apply. GUI’s seemed fixed. A number of files were updated in my home directory (find -newermt "1 minute ago"). I think there are a number of changes going on in this area.
Regarding sound, check the KDE Bug Database. They seem to point to either the kernel or pulseaudio. There are entries in the forum, like: