System theme settings are not applied

Default installation does not retain system level theme settings, e.g. the dark mode or high contrast theme. This is an issue in the Flatpak sandboxing that does not allow even real only access to system theme folders and user's settings files.

Manually copying system themes to `~/.themes` folder (`cp -r /usr/share/themes/ ~/.themes/`) and giving Flatpaks access to this folder with `flatpak override --user --filesystem=~/.themes` is a workaround for this issue, but a usable out of the box solution would be preferable.

Issue initially reported in https://github.com/chidiwilliams/buzz/issues/1096