In order for Flatpak'd applications to integrate with the system they require having themes installed but sadly this has always been a manual process. Thankfully Flatpak has a feature for basic conditional statements in runtimes for automatically downloading or enabling extensions if a condition is met. These conditions are defined within flatpak itself so I added active-gtk-theme.
This condition currently pulls from DConf looking up the gtk-theme key in the org.gnome.desktop.interface schema which is set automatically by GNOME and thanks to a recent addition KDE.
On other platforms it would be necessary to pull the information from XSettings but that is sadly a non-trivial protocol and would likely mean a dependency on xlib which is not desired so has not been implemented at this time.
Once the condition is met at update or install time flatpak will automatically install matching extensions.
Merge request: https://github.com/flatpak/flatpak/pull/1181 and
https://github.com/flatpak/freedesktop-sdk-images/pull/60
