Oh no something has gone wrong. A problem has occurred and the system can't recover. How to roll back Ubuntu 20.04 to post-install state?

Question

linux

I recently upgraded from Ubuntu 16 to 20. I wanted to change the background of the authorization window, I found a command on the Internet, but everything broke because of it. I fixed it with the help sudo dpkg --congigure -ain safe mode, but it was not completely fixed.
I found a command to reset everything back, but the system reset to the 16th interface, and on the splash screen there is a panther on a pink background. I set the Yaru-dark theme in the settings, but after the reboot everything broke completely.

After entering the password (the authorization window is the same unpleasant panther), the desktop does not open, but the error
Oh no something has gone wrong pops up. A problem has occurred and the system can't recover.

I went to the terminal and tried all the following:

sudo apt-get update
sudo apt-get upgrade
sudo apt install --reinstall ubuntu-desktop

and a few more commands from the Internet. Nothing helps. Help please roll back the system back!