Can't reproduce on Neon with 5.11.9. Did you restart any GTK programs after changing the setting? Can you provide detailed steps to reproduce, or even better, a screen recording depicting the issue?  SimpleScreenRecorder works great for this.

*** Bug 385791 has been marked as a duplicate of this bug. ***

Re-opening because we have another report of the same issue, so even if I can't reproduce it, there's probably *something* wrong here.

I don't really know whether the bug I filed (Bug 385791) is really a duplicate of this. The initial description is not detailed. And in the case of Bug 385791 GTK application style *does get changed* indeed. The problem is when you want to open again the kde-gtk-config kcmshell module, the values of the *module* are all reset and do not reflect what you had changed. So please reconsider marking Bug 385791 as a duplicate of this.

On the matter of other reports, all Slackware users using Plasma 5.11 are reporting the issue. See comments at this thread:

https://alien.slackbook.org/blog/october-updates-for-the-slackware-plasma5-desktop/

(I don't know how to link to individual comments to spare you the search, sorry about that. Search "gtk" and you'll find the relevant data). Thanks!

Despite the vagueness, I'm pretty sure this bug is reporting the same issue, btu I'll reverse the dupe order since your bug is more detailed. Mind migrating that link over to the other bug?

*** This bug has been marked as a duplicate of bug 385791 ***

Yes, it is the same as  bug 385791.
I only realised after reporting that just the kde-gtk-config kcmshell module get reset. 
(As before, I tested the effect of changing values by opening libreoffice without noticing that libreoffice-gtk3 package was missing, so there would apparently be no effect.)

*** This bug has been marked as a duplicate of bug 385776 ***

*** This bug has been marked as a duplicate of bug 385791 ***

*** This bug has been marked as a duplicate of bug 382291 ***