I just avoid GTK apps as much as possible, thankfully the few I do use look decent enough but only because they follow darkmode atleast.
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Plasma 6 Bugs
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I made some notes while I had a problem with Zim flatpak not following Breeze GTK theme:
https://docs.flatpak.org/en/latest/desktop-integration.html#applying-themes //The applications will try to match the system theme currently being used, if it corresponds to any of the Flatpak themes installed, and will fall back to Adwaita (if they use Gtk2 or Gtk3) or the default Qt theme (if they use Qt) if a corresponding theme isn’t detected.//
== Solution: ==
''flatpak override --user --filesystem=xdg-config/gtk-3.0:ro''
Source: https://github.com/flathub/org.gtk.Gtk3theme.Breeze#workarounds
( I tried to install ''org.gtk.Gtk3theme.Breeze-Dark/x86_64/3.22'' but //"This theme has been replaced by org.gtk.Gtk3theme.Breeze, see README for workaround on using system color schemes. https://github.com/flathub/org.gtk.Gtk3theme.Breeze#workarounds"////////
Context: Kubuntu 22.04 KDE Plasma 5.24.7