To install Flatpak, run the following:
sudo apt install flatpak
For Debian Jessie and Stretch, a flatpak package is available in the official backports repository.
Flathub is the best place to get Flatpak apps. To enable it, run:
flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
To install light and dark themes, run:
flatpak install flathub org.gtk.Gtk3theme.deepin
flatpak install flathub org.gtk.Gtk3theme.deepin-dark
To complete setup, restart your system. Now all you have to do is install some apps!
Note: graphical installation of Flatpak apps may not be possible with Alpine.