Automatic theme extension installation doesn't work with GNOME 3.30 runtime
As per https://github.com/flatpak/flatpak/issues/2368
If the user installs the GNOME 3.30 runtime as the first runtime, any non-Adwaita themes are not automatically installed.
TingPing on IRC suggests that the meta needs the following "the Platform and Sdk extension point for gtk3theme needs to add download-if: active-gtk-theme
" - eg in the following place https://gitlab.gnome.org/GNOME/gnome-build-meta/blob/master/elements/flatpak/platform.bst#L58 and in the SDK.
Edited by Andrew Hayzen