Why do some applications not receive GTK stylings? E.g. Thunderbird

Some applications do not get styled in the regular system GTK theme, but instead show rather weird appearance. I installed Thunderbird 115 via snap package first and saw a rather ugly interface. I had assumed not to have installed all the snap packages related to GTK on this computer, thus my lazy shortcut was to just go with the deb version of TB instead. Still, same issue. What strikes me odd is that not even when manually setting the icon theme to e.g. Papirus (added via PPA) changed anything. I made sure to activate the "systems theme" in TB itself to no avail. I do not see the same behaviour on my other computers that run regular Ubuntu 22.04 with GTK 42.9, neither in snap nor in deb versions of software packages. For example, when I use a different GTK theme that is not available in certain snap packages, they tend to fall back to Yaru from the gtk-common-themes snap package.

ZorinOS 17.1 with TB 115 dark

Zorin with TB light

Ubuntu 22.04 with Graphite GTK theme and Papirus icon theme

Ubuntu 22.04 with Yaru theme

I especially do not understand why the icons get ignored (check the left sidebar, or the reply buttons). I had tried to debug with looking-glass, but no idea what is going wrong here.

Snap's and Flatpak's don't use a System Theme because there are running in Container's. So, they don't have access to the whole System. When You use a .deb Version it should work without Problems.

Where do You have placed Your Theme and Icon Folders?

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I think that Flatpak applications under GNOME can be made to follow system themes:

itsfoss: Apply GTK Themes on Flatpak Apps

Yes, it is possible to make it. But by default they will not take the Theme.