Hello!

Since the update to Plasma 5.19 the themes for GTK 2 applications do not work anymore (the Raleigh theme is used by default independently of our choices and preferences). I've done some testing, uninstalling (with "apt remove --purge") and installing again the theme doesn't work. However, if you try to launch a GTK 2 app when previewing a GTK 2 theme (in the Plasma control panel), it will launch correctly with this theme applied. But as soon as you close the preview window, the app reverts back to the default theme (sic!)...

GTK 3 themes seem to work properly.

I've reproduced the issue both at my PC at home (which uses KDE Neon as the only operating system) and in the Virtual Box on the iMac I have at work.

I'm crossing fingers for the KDE team to solve this issue.

It could help if you mention the Apps that you have a specific problem with.

I've seen it with all the GTK2 apps I use regularly - which mean GIMP, Inkscape and Ripperx...

No such problem here. Newly installed user edition with plasma 5.19. I installed Gimp & it follows the Breeze them fine. I even downloaded a couple of GTK2/3 themes, & Gimp follows them. Leads me to believe that it is probably some configuration file or some corrupt file left over from your update from 5.18.
It would be interesting to see if anyone else has the issue, although no one else has posted.

Probably an issue with the update then - as I can reproduce the problem on two independent installations (one on a PC, heavily used daily, and one in a Virtual Box on iMac, used for testing)... If only I knew which configuration files modify or delete, it would be wonderful...

And I'm not the only one with the issue: viewtopic.php?f=309&t=166475

I've reported an official bug: https://bugs.kde.org/show_bug.cgi?id=423141