I am running Sway on Debian 12 and I just can’t succeed in theming QT Flatpaks. The system is in a dark theme but all the QT Flatpaks are using a light theme. I tried a bunch of overrides ans exposes the .themes folder in my home after copying all the themes. Nothing seems to work. Did anyone succeed in theming these apps in a window manager?
The themes section on the desktop integration doc here might be of use: https://docs.flatpak.org/en/latest/desktop-integration.html
This may be useful for further elaboration: https://www.reddit.com/r/flatpak/comments/y9jmqj/the_general_flatpak_qt_and_gtk_theming_guide/
Going off of these guides my best guess would be that you don’t yet have kvantum or some other theme runtime installed within flatpak
I’ll also mention Stylepack exists for automating and streamlining theme installation, it’s only for GTK currently however but maybe will be of use in future: https://github.com/refi64/stylepak