Flatpak - "dark theme" icons missing

[Jopp-gh]

Since a month or two, all my flatpaks window manager button icons (minimize, zoom, close) have disappeared. Luckily, wm buttons work still as expected on click, just their icons are missing:

I get only gray wm-buttons without icons, so my question are:

• what dark flatpak themes are needed on Mint?
• do I have some flatpak theme folder/file with wrong permissions?
• where are dark flatpak themes stored?
• how to debug missing flatpak resources?

Probably I mistakenly uninstalled one of many Mint themes shipped by default, but needed for flatpak. And I thought, flatpaks are self contained and independent from outside resources. It seems not.

I know, flatpak need Adwaita and gtk4 to work, but this doesn't help me further to correct missing icons.