Ubuntu Mate Linux 20.04 comes with Mate 1.24.0
Ubuntu Mate Linux 22.04 comes with Mate 1.26.0
In Control Panels -> Appearance the themes are not the same between the two versions of Mate.
On Mate 1.24.0 I settled on Ambiant-MATE which no longer exists on Mate 1.26.0. On Mate 1.26.0 I tried Yaru-MATE-dark BUT all the colors are so dark that when you have many open windows it is difficult to distinguish the window title bars from the windows background. This results in too many mouse clicks in the dark so to speak. I like clear distinct colors between window title bars and contents.
Correct Solution
# https://github.com/lah7/Ambiant-MATE
sudo add-apt-repository ppa:lah7/ambiant-mate
sudo apt install ambiant-mate-gtk-themes ambiant-mate-icon-themes
# Open the Control Panels -> Appearance pannel: you can now switch to Ambiant-MATE
Ad Hoc Stupid Solution...
...which I first used by digging into the file system after 2+ hours of unsuccessful googling (I guess I was not using the right keywords).
# In a terminal on Ubuntu Mate Linux 20.04:
cd /usr/share/icons
sudo tar -cjf Ambiant-MATE-icons.tar.bz2 Ambiant-MATE
sudo mv Ambiant-MATE-icons.tar.bz2 ~/Downloads
sudo chown domih.domih ~/Downloads/Ambiant-MATE-icons.tar.bz2
cd /usr/share/themes
sudo tar -cjf Ambiant-MATE-themes.tar.bz2 Ambiant-MATE
sudo mv Ambiant-MATE-themes.tar.bz2 ~/Downloads
sudo chown domih.domih ~/Downloads/Ambiant-MATE-themes.tar.bz2
# Copy the two tar balls to your Ubuntu Mate Linux 22.04 installation (via Samba, scp, USB key, etc)
# In a terminal on Ubuntu Mate Linux 22.04:
sudo mv ~/Downloads/Ambiant-MATE-icons.tar.bz2 /usr/share/icons
cd /usr/share/icons
sudo tar -xf Ambiant-MATE-icons.tar.bz2
sudo rm Ambiant-MATE-icons.tar.bz2
sudo mv ~/Downloads/Ambiant-MATE-themes.tar.bz2 /usr/share/themes
cd /usr/share/themes
sudo tar -xf Ambiant-MATE-themes.tar.bz2
sudo rm Ambiant-MATE-themes.tar.bz2
CONCLUSION: use the correct solution
