I'm having trouble getting Flatpak GTK4 applications to display correctly on my system. As shown in the screenshot below, there is a clear mismatch between the two themes, and I'd like the Flatpak version to appear properly as it does on the right.

Theme mismatch between the Flatpak and native versions of an example GTK4 app, Blanket.

I'm assuming this is most likely an issue with my Arch system and not Flatpaks in general, as Flathub screenshots display properly themed GTK4 applications that upon installing on my system, look nothing like these screenshots. I'd also like to note that I am currently using KDE Plasma as my desktop environment, but these issues seem to persist across DE's. I tested on GNOME as well.

The solution I am not looking for is to simply use the Arch equivalent packages or give up on Flatpaks. I want the Flatpaks to be themed properly on my system. This is not an exclusively stylistic issue either, it's also a usability one. "Snackbar"-like popups are missing backgrounds, and padding is missing in many places, both problems significantly decreasing legibility of the interfaces.

The lack of a backdrop, drop-shadow, and padding in general makes these about dialogs particularly troublesome to read. A good example of the issue, but by far not the only one.

Thank you in advance to all help available.