Okay - update for lost souls like me
That ZorinOS 17 is not officially released for 16.3 Education
You can get it, if you bought "Pro" (like I did).
Install the latest updates from the Software Updater
Open the Terminal (Ctrl+Alt+T) and enter this command: gsettings set com.zorin.desktop.upgrader show-test-upgrades true
Open the Zorin menu > search & open "Upgrade Zorin OS"
After the Upgrade Zorin OS app finishes checking for upgrades, click the "Upgrade" button beside the "Zorin OS 17 Pro" option.
Follow the on-screen instructions and enter your order details for Zorin OS 17 Pro when prompted.
BUT! ATTENTION!
That update messed up my install. After fiddling around a bit, I managed to get the standard Gnome desktop. So I was able to work again.
Here is, what solved my problem:
What I did:
Edit /etc/apt/sources.list and uncommented eveything except:
deb Index of /ubuntu jammy-backports main restricted universe multiverse
deb-src Index of /ubuntu jammy-backports main restricted universe multiverse
deb Index of /ubuntu jammy partner
deb-src Index of /ubuntu jammy partner
deb Index of /ubuntu jammy main restricted universe multiverse
deb Index of /ubuntu jammy-updates main restricted universe multiverse
deb Index of /ubuntu jammy-security main restricted universe multiverse
Then I ran the usual sudo apt update - upgrade
After that, I dropped the following install line:
sudo apt install ukui-control-center ukui-desktop-environment ukui-desktop-environment-core ukui-desktop-environment-extras ukui-media ukui-media-common ukui-menu ukui-menus ukui-panel ukui-power-manager ukui-screensaver ukui-screensaver ukui-session-manager ukui-settings-daemon ukui-settings-daemon-common ukui-settings-daemon ukui-window-switch -y
After the first boot after that, my keyboard and mouse were frozen, so one reboot in ZorinOS options --> generic and after that, everything worked as expected.
Booted again and back to "normal" ZorinOS.
Now I was able to switch to Zorin-Look (Logon-Screen, click name, click gear-icon on the lower right).
Systemtools - Zorin Appearance works now. Everything seems to be fine.