After Plasma got updated from 5.18.4 to 5.18.5 in Kubuntu 20.04, flatpak apps don't honor the host theme anymore. I also tested this under KDE Neon User edition. xsettingsd is running.
Cannot reproduce on Kubuntu 20.04, but xsettingsd (which is responsible for flatpak apps on X11) can be buggy sometimes. I suggest you to restart it a couple of times or to set the theme back and forth a couple of times.
Restarting xsettingsd or changing the theme does indeed make it apply the theme to flatpak apps, but after a logout the effect is gone and you have to redo it everytime. I wouldn't consider this " WORKSFORME", and it wasn't like this in 5.18.4.
Still cannot reproduce. There were no changes in GTK intergation codebase in 5.18.5. What GTK theme are you applying?
I'm switching between Breeze and Breeze-Dark. How quick I login again, seems to play a role. When I fix the flatpak theme (by switching back and forth between Breeze and Breeze-Dark), then logout and I wait some seconds before the next login, I can get it to save the settings for flatpak apps for the next three logins (it always stops remembering after that). After a quick re-login, it never works correctly (the theme doesn't get applied to flatpak apps).
In the next version of Plasma (5.19) there is no option of Breeze-Dark scheme in the list any longer, since the Breeze grabs its colors from the color scheme. Can you reproduce the issues with other themes?
I was using "Global Theme" for switching between Breeze and Breeze-Dark. But using "Application Style > GNOME/GTK Application Style" to switch between e.g. "Breeze" and "Emacs" has the same effect: flatpak apps have the correct theme till the next login.
Can you confirm, that that only happens with Emacs and Breeze-Dark GTK themes? Are any other themes causing this issues?
I can't reproduce this on another installation of Kubuntu 20.04, so closing this. Must be some very obscure interaction on that other system. > In the next version of Plasma (5.19) there is no option of Breeze-Dark scheme in the list any longer, Going to "Application Style > GNOME/GTK Application Style" and setting Breeze-Dark explicitly there, was the only why I could get Flatpak apps to apply the dark theme. Have you tested this?
> Going to "Application Style > GNOME/GTK Application Style" and setting > Breeze-Dark explicitly there, was the only why I could get Flatpak apps to > apply the dark theme. Have you tested this? Flakpak apps indeed do not folow the system color scheme. Feel free to open a separate bug for that.