It's exactly like when I tried to create a new user from the root console in recovery mode : the error screen changed theme (reverting to the default white one) but the error remains.
I don't remember doing anything about a version update to do that. I just tried the commands since the error seemed close to the issue I had and I feared there was an automatic update in a half-finished status (which is not the case since I was able to verify later I was still on Core 17.3).
I just did what I always do when booting my computer, which is updating everything and go on for the day.
the update routine
sudo apt update -y && sudo apt upgrade -y && sudo apt autoremove -y && sudo apt clean -y && sudo apt autoclean -y
sudo snap refresh
brew update && brew outdated && brew upgrade && brew cleanup
I finished the day with some experiments on adding an option on the file explorer's right click, but as I just added a file to ~/.local/share/nautilus/scripts, I doubt it had such an effect on the whole OS.