Found the theming thing: The magic is done by having the QGnomePlatform plugin installed. On Debian based systems the package should be qgnomeplatform-qt5. Then launching Picard with QT_QPA_PLATFORMTHEME=gnome would use the color scheme set there. When running actually GNOME this would already be set automatically. Dark / light theme detection requires QGnomePlatform >= 0.9.
That won’t work with the AppImage though, unless it would have the QGnomePlatform plugin bundled. Which I won’t do. The AppImage currently uses the Qt5 binaries provided by PyQt5 from PyPI, and QGnomePlatform needs to be compiled against the proper Qt version.
That would mean essentially fully compiling Qt5 and PyQt5 for the package, and I have zero interest investing any time doing so. Also QGnomePlatform seems to be no longer maintained.
With Qt6 things will change anyway.