I know Zorin 16 is still in beta, I'm posting this here not only to find out a solution, but to also help others who might have the same problem. I already also e-mailed this through proper feedback channels for Zorin 16.
zorin-appearance does not open anymore and this is the output if I try to run it via terminal (with or without sudo).
WARNING:root:Shell not installed or running
Traceback (most recent call last):
File "/usr/bin/zorin-appearance", line 11, in <module>
load_entry_point('zorin-appearance==4.0', 'gui_scripts', 'zorin-appearance')()
File "/usr/lib/python3/dist-packages/pkg_resources/__init__.py", line 490, in load_entry_point
return get_distribution(dist).load_entry_point(group, name)
File "/usr/lib/python3/dist-packages/pkg_resources/__init__.py", line 2854, in load_entry_point
return ep.load()
File "/usr/lib/python3/dist-packages/pkg_resources/__init__.py", line 2445, in load
return self.resolve()
File "/usr/lib/python3/dist-packages/pkg_resources/__init__.py", line 2451, in resolve
module = __import__(self.module_name, fromlist=['__name__'], level=0)
File "/usr/lib/python3/dist-packages/zorin_appearance/__main__.py", line 20, in <module>
from zorin_appearance import theme
File "/usr/lib/python3/dist-packages/zorin_appearance/theme.py", line 24, in <module>
from zorin_appearance import environment
File "/usr/lib/python3/dist-packages/zorin_appearance/environment.py", line 20, in <module>
_desktop_environment = os.environ['XDG_CURRENT_DESKTOP']
File "/usr/lib/python3.8/os.py", line 675, in __getitem__
raise KeyError(key) from None
KeyError: 'XDG_CURRENT_DESKTOP'
Reinstalling the packages via Synaptic didn't work.
Thanks.