On Kali Linux, flatpak can be installed through:

[email protected]:~$ sudo apt update
[email protected]:~$
[email protected]:~$ sudo apt install -y flatpak
[email protected]:~$
[email protected]:~$ sudo flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo

It is also a good idea to install the Flatpak plugin for GNOME Software. To do this, run:

[email protected]:~$ sudo apt install gnome-software-plugin-flatpak

Now you can install new flatpak apps from the software center.

If you want to make flatpak apps look more consistent with the system you can force them to use your local themes:

[email protected]:~$ mkdir -p ~/.themes
[email protected]:~$ cp -a /usr/share/themes/* ~/.themes/
[email protected]:~$ sudo flatpak override --filesystem=~/.themes/

• Flatpak
• Debian Quick Setup