If that helps, then icon theme is not applied also to KDE4 apps eg. okular. The style is applied though.
So maybe kde-gtk-config is not "guilty" in that at all...

Some update yesterday evening seems to fix that problem - firefox and thunderbird are using breeze icons again. So closing that bug for now...

Status: NEW => RESOLVED
Resolution: (none) => FIXED

Made a fresh install and the bug is indeed there.
There is .gtkcr-2.0 file in home folder and files gtkrc and gtkrc-2.0 in .config folder.
If I delete all of them and create new files with kde-gtk-config app in systemsettings, then also the icon theme for gtk apps will change to the selected one there and stays so...

Resolution: FIXED => (none)
Status: RESOLVED => REOPENED

Isn't it normal that the specified theme is not applied to Okular? Okular is a Qt application AFAIK.

Based on comment 3, I guess the issue happens only when there is some old config laying around?

Blocks: (none) => 17523

Made a Cauldron network install January 20th by minimal install with X + later urpmi --no-recommends task-plasma5-minimal
Installing breeze-gtk theme and applying it also with breeze icon theme using kde-gtk-config kcm app made gtk apps use breeze icons.
So I guess we can close this bug.

Resolution: (none) => FIXED
Status: REOPENED => RESOLVED

Not fixed for me (Mageia 6)

How reproducible:every time

Steps to Reproduce:
1) Launch fresh Plasma (e.g. Mageia-6-LiveDVD-Plasma-x86_64-DVD)
2) Go to System Settings - Application Style - GNOME Application Style (GTK) - select "Icon theme: Adwaita"
3) Launch GIMP - open  <File> menu - see Breeze icons
4) Log out - log in - nothing changes
5) Go System Settings - Workspace Theme - select Breeze Dark - Log Out
6) Log In - launch GIMP, now it uses Breeze Dark icon theme

Resolution: FIXED => (none)
Status: RESOLVED => UNCONFIRMED
CC: (none) => altsyde
Ever confirmed: 1 => 0

Here's what I found:

1) System icon settings have highest priority, Adwaita icon theme is on the second place, icon theme you had set on "GNOME Application Style" on vanilla Mga6 installation/live is completely ignored
How to check:
a) System settings - Icons - select - Oxygen.
b) Go to System Settings - Application Style - GNOME Application Style - select Breeze. Log out - Log in
c) Launch GIMP just to make sure it still uses Oxygen. Close it
d) in konsole:
su
# cd /usr/share/icons
# mv oxygen 1xygen
don't close terminal
e) Launch GIMP, now it uses Adwaita (not Breeze!)
f) # rm Adwaita
Now GIMP uses other icons, GTK file dialogs doesn't have icons. So Breeze is really ignored.

2) Workspace Theme also forces icons to GTK applications
How to check:
a) Go to System Settings - Workspace Theme - Look and Feel - select Breeze Dark
b) Return to All Settings, on Application Style - GNOME Application Style click "Default" button, selected "Icon theme" changes to Breeze Dark.

I don't really mind Breeze icons in GTK applications but my problem is that if I'm using Dark theme for Plasma and Light theme for GTK apps those icons are hardly visible. So my current workaround is to apply Breeze Dark separately:
a) Leave Workspace theme to Mageia, change Desktop theme to Breeze Dark
b) All Settings - Color -Breeze Dark - uncheck "Apply colors to non-QT applications"

is this issue still valid ?

There are some upstream changes in Plasma with kde-gtk-config

$ urpmq -i kde-gtk-config
kde-gtk-config-5.20.1-1.mga8.src.rpm

Now, default Settings in Cauldron is that it selects theme Adwaita by default to all GTK app. Libreoffice uses other type of integration and under Plasma it uses VLC-kf5 which gives a Breeze theme.

Selecting "Breeze" applies current Breeze selection in other KCM.

Closing this.

Resolution: (none) => WORKSFORME
Source RPM: kde-gtk-config-5.4.0-1.mga6.src.rpm => kde-gtk-config-5.20.1-1.mga8.src.rpm
Status: UNCONFIRMED => RESOLVED