I switched to Feren OS recently that uses Plasma Display by KDE. I've set a dark theme in the global themes option. The problem is that applications like Gimp and Inkscape doesn't get dark. The 'use dark variant' checkbox does nothing which I guess should use dark theme as it does in Windows. System apps are dark. Libre Office that was pre-installed is also dark. I've tried many things but all failed. Installed Gnome-tweaks but it does nothing, can't even change icons.
Any help would be appreciated.
Feren Os is based on Ubuntu LTS.